Congrats! You just learned how to create your custom sword in the last tutorial! Now imagine you want to shield yourself from that sword if your friend got it. If you made it too op, a regular shield won't do. So we will see how to make a custom shield.
Luckily, StellarWind22 has already made a library to help with this! If she didn't, you probably would be sitting here for the next hour following this tutorial, so thanks StellarWind22!

The library source is available at https://github.com/CrimsonDawn45/Fabric-Shield-Lib

Library compiled as a jar is available at https://www.curseforge.com/minecraft/mc-mods/fabric-shield-lib

Add the following code to the files mentioned:

gradle.properties

fabric_shield_lib_version=1.7.2-1.20.4
midnightlib_version=1.5.2-fabric
mod_menu_version=9.0.0-pre.1
fabricasm_version=2.3

build.gradle (under dependencies)

	modImplementation "com.github.CrimsonDawn45:Fabric-Shield-Lib:v${project.fabric_shield_lib_version}"
 
	modImplementation "com.terraformersmc:modmenu:${project.mod_menu_version}"
 
	modImplementation "maven.modrinth:midnightlib:${project.midnightlib_version}"
 
	modImplementation "com.github.Chocohead:Fabric-ASM:v${project.fabricasm_version}"

At the time of writing, the latest project.fabric_shield_lib_version should be 1.7.2, which supports versions:

• 1.20.2 - 1.20.4 (1.7.2-1.20.4)
• 1.20 - 1.20.1 (1.7.2-1.20.1)
• 1.19.3 - 1.19.4 (1.7.2-1.19.4)
• 1.19 - 1.19.2 (1.7.2-1.19.2)
• 1.18.2 (1.7.2-1.18.2)
• 1.17.1 (1.7.2-1.17.1)
  

build.gradle (inside repositories, the one above dependencies)

	maven {url = 'https://jitpack.io'}
	maven {url "https://maven.terraformersmc.com/releases/"}
	maven {url = "https://api.modrinth.com/maven"}

Midnight Lib and FabricASM are included in the FabricShieldLib release (.jar on Curseforge/Modrinth) but need to be used as dependencies when developing

If you want your shield to support banner decoration on your shield, please skip to the next section
On to the non-boring steps! We will now make a custom shield!

If you have followed the above steps correctly and refreshed the project, then you will have the FabricShieldLib installed.
If so, the first step to do is create a new instance of an Item like:

public static final Item NETHERITE_SHIELD = new FabricShieldItem(new FabricItemSettings().maxDamage(2500), 10, 13, Items.NETHERITE_INGOT); // FabricShieldItem(settings.maxDamage(durability), cooldownTicks, enchantability, repairItems)

Then, we have to register it, like so:

Registry.register(Registries.ITEM, new Identifier("examplemod", "netherite_shield"), NETHERITE_SHIELD);

If you want to add your shield to a creative tab, add this into your onInitialize() method:

ItemGroupEvents.modifyEntriesEvent(ItemGroups.COMBAT).register(entries -> {
	entries.add(NETHERITE_SHIELD);
});

And our shield is (code-wise) done!
Now, we have to create the textures and models of the shield.
For the texture, you can use anything. A good place to start is to look at Mojang's shield texture and change it. Put it in resources/assets/examplemod/textures/item/netherite_shield.png
Now, for the models, we have to write a few .json files.

Inside resources/assets/examplemod/models/item/, create a netherite_shield.json file and put this inside it:

{
    "parent":"fabricshieldlib:item/fabric_shield",
    "textures":{
        "shield":"examplemod:item/netherite_shield"
    },
    "overrides": [
        {
            "predicate": {
                "blocking": 1
            },
            "model": "examplemod:item/netherite_shield_blocking"
        }
    ]
}

In the same folder, create another file, netherite_shield_blocking.json, and put his inside it:

{
  "parent": "fabricshieldlib:item/fabric_shield_blocking",
  "textures":{
    "shield":"examplemod:item/netherite_shield"
  }
}

Lastly, create a shields.json file in resources/data/c/tags/items/shields.json and add your shield to it:

{
  "replace": false,
  "values": [
    "examplemod:netherite_shield"
  ]
}

Don't forget to add it to en_us.json in resources/assets/lang/en_us.json

{
  "item.examplemod.netherite_shield": "Netherite Shield"
}

And with that, your non-decorateable shield is done!

The process is slightly different if you want to make your shield accept banners and decorateable like a vanilla shield.

We still make the item in the same way, just make it a FabricBannerShieldItem:

public static final Item NETHERITE_BANNER_SHIELD = new FabricBannerShieldItem(new FabricItemSettings().maxDamage(2500), 10, 13, Items.NETHERITE_INGOT); // FabricBannerShieldItem(settings.maxDamage(durability), cooldownTicks, enchantability, repairItems)

Then, we have to register it:

Registry.register(Registries.ITEM, new Identifier("examplemod", "netherite_banner_shield"), NETHERITE_BANNER_SHIELD);

If you want to add your shield to a creative tab, add this into your onInitialize() method:

ItemGroupEvents.modifyEntriesEvent(ItemGroups.COMBAT).register(entries -> {
	entries.addAfter(Items.SHIELD,NETHERITE_BANNER_SHIELD);
});

Now the item is created, we need to set up its rendering. For these steps, we are going to be working in our client mod initializer, which should already be set up for you in the fabric example mod.

First, we will need to create an EntityModelLayer to use later.

public class ExampleModClient implements ClientModInitializer {
 
    public static final EntityModelLayer netherite_banner_shield_model_layer = new EntityModelLayer(new Identifier("examplemod", "netherite_banner_shield"),"main");
 
    @Override
    public void onInitializeClient() {
 
    }
}

Next, we will register out EntityModelLayer that we made previously.

public class ExampleModClient implements ClientModInitializer {
 
    public static final EntityModelLayer NETHERITE_SHIELD_MODEL_LAYER = new EntityModelLayer(new Identifier("examplemod", "netherite_shield"),"main"); ​
 
    @Override
    public void onInitializeClient() {
	EntityModelLayerRegistry.registerModelLayer(netherite_banner_shield_model_layer, ShieldEntityModel::getTexturedModelData);
    }
}

Then we will create our shield model,

public class ExampleModClient implements ClientModInitializer {
 
    public static final EntityModelLayer NETHERITE_SHIELD_MODEL_LAYER = new EntityModelLayer(new Identifier("examplemod", "netherite_shield"),"main"); ​
 
    public static ShieldEntityModel modelNetheriteShield;
 
    @Override
    public void onInitializeClient() {
        EntityModelLayerRegistry.registerModelLayer(NETHERITE_SHIELD_MODEL_LAYER, ShieldEntityModel::getTexturedModelData);
    }
}

And then register that shield model,

public class ExampleModClient implements ClientModInitializer {
 
    public static final EntityModelLayer NETHERITE_SHIELD_MODEL_LAYER = new EntityModelLayer(new Identifier("examplemod", "netherite_shield"),"main"); ​
 
    public static ShieldEntityModel modelNetheriteShield;
 
    @Override
    public void onInitializeClient() {
        EntityModelLayerRegistry.registerModelLayer(NETHERITE_SHIELD_MODEL_LAYER, ShieldEntityModel::getTexturedModelData);
 
        ShieldSetModelCallback.EVENT.register((loader) -> {
	    modelNetheriteShield = new ShieldEntityModel(loader.getModelPart(netherite_banner_shield_model_layer));
	    return ActionResult.PASS;
	});
    }
}

Next, we have to create our two SpriteIdentifiers,

public class ExampleModClient implements ClientModInitializer {
 
	public static final EntityModelLayer netherite_banner_shield_model_layer = new EntityModelLayer(new Identifier("examplemod", "netherite_banner_shield"),"main");
 
	public static ShieldEntityModel modelNetheriteShield;
 
	public static final SpriteIdentifier NETHERITE_BANNER_SHIELD_BASE = new SpriteIdentifier(SpriteAtlasTexture.BLOCK_ATLAS_TEXTURE, new Identifier("examplemod", "entity/netherite_banner_shield_base"));
	public static final SpriteIdentifier NETHERITE_BANNER_SHIELD_BASE_NO_PATTERN = new SpriteIdentifier(SpriteAtlasTexture.BLOCK_ATLAS_TEXTURE, new Identifier("examplemod", "entity/netherite_banner_shield_base_nopattern"));
 
	@Override
	public void onInitializeClient() {
            EntityModelLayerRegistry.registerModelLayer(NETHERITE_SHIELD_MODEL_LAYER, ShieldEntityModel::getTexturedModelData);
 
            ShieldSetModelCallback.EVENT.register((loader) -> {
	        modelNetheriteShield = new ShieldEntityModel(loader.getModelPart(netherite_banner_shield_model_layer));
	        return ActionResult.PASS;
	    });
	}
}

Finally, we are going to register our shield with the BuiltinItemRendererRegistry,

public class ExampleModClient implements ClientModInitializer {
 
	public static final EntityModelLayer netherite_banner_shield_model_layer = new EntityModelLayer(new Identifier("examplemod", "netherite_banner_shield"),"main");
 
	public static ShieldEntityModel modelNetheriteShield;
 
	public static final SpriteIdentifier NETHERITE_BANNER_SHIELD_BASE = new SpriteIdentifier(SpriteAtlasTexture.BLOCK_ATLAS_TEXTURE, new Identifier("examplemod", "entity/netherite_banner_shield_base"));
	public static final SpriteIdentifier NETHERITE_BANNER_SHIELD_BASE_NO_PATTERN = new SpriteIdentifier(SpriteAtlasTexture.BLOCK_ATLAS_TEXTURE, new Identifier("examplemod", "entity/netherite_banner_shield_base_nopattern"));
 
	@Override
	public void onInitializeClient() {
            EntityModelLayerRegistry.registerModelLayer(NETHERITE_SHIELD_MODEL_LAYER, ShieldEntityModel::getTexturedModelData);
 
            ShieldSetModelCallback.EVENT.register((loader) -> {
	        modelNetheriteShield = new ShieldEntityModel(loader.getModelPart(netherite_banner_shield_model_layer));
	        return ActionResult.PASS;
	    });
 
            BuiltinItemRendererRegistry.INSTANCE.register(ExampleMod.NETHERITE_BANNER_SHIELD, (stack, mode, matrices, vertexConsumers, light, overlay) -> {
		renderBanner(stack, matrices, vertexConsumers, light, overlay, modelNetheriteShield, NETHERITE_BANNER_SHIELD_BASE, NETHERITE_BANNER_SHIELD_BASE_NO_PATTERN);
                //The first five parameters are taken from the method, while the last 3 you provide yourself. You will provide the model, and then your 2 sprite identifiers in the order of SHIELD_NAME_BASE and then SHIELD_NAME_BASE_NOPATTERN.
	    });
	}
}

That is all of our code done, we only have a few .json files to make.

First, create a blocks.json file inside of resources/assets/minecraft/atlases/.

Inside of it, you will have to list your SpriteIdentifiers from your client mod initializer like this:

{
  "sources": [
    {
      "type": "single",
      "resource": "examplemod:entity/netherite_banner_shield_base"
    },
    {
      "type": "single",
      "resource": "examplemod:entity/netherite_banner_shield_base_nopattern"
    }
  ]
}

Next, inside resources/assets/examplemod/models/item/, create a netherite_banner_shield.json file and put this inside it:

{
  "parent":"fabricshieldlib:item/fabric_banner_shield",
  "overrides": [
    {
      "predicate": {
        "blocking": 1
      },
      "model": "examplemod:item/netherite_banner_shield_blocking"
    }
  ]
}

In the same folder, create another file, netherite_banner_shield_blocking.json, and put his inside it:

{
  "parent":"fabricshieldlib:item/fabric_banner_shield_blocking",
  "textures":{
    "shield":"examplemod:item/netherite_shield"
  }
}

For this next step, you will add a netherite_shield_base_nopattern texture, which will be a standard shield texture. Then, you will need to make a base version for your shield texture, so netherite_shield_base. I recommend looking at the vanilla shield's base texture.

Then, you will move both of these textures into resources/assets/<modid>/textures/entity.

Lastly, create a shields.json file in resources/data/c/tags/items/shields.json and add your shield to it:

{
  "replace": false,
  "values": [
    "examplemod:netherite_banner_shield"
  ]
}

Don't forget to add it to en_us.json in resources/assets/lang/en_us.json along with the color variants,

{
  "item.examplemod.netherite_banner_shield": "Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.red": "Red Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.orange": "Orange Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.yellow": "Yellow Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.lime": "Lime Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.green": "Green Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.light_blue": "Light Blue Banner Netherite Shield",
  "item.examplemod.netherite_banner_shield.cyan": "Cyan Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.blue": "Blue Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.purple": "Purple Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.magenta": "Magenta Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.pink": "Pink Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.brown": "Brown Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.white": "White Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.light_gray": "Light Gray Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.gray": "Gray Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.dark_gray": "Dark Gray Netherite Banner Shield",
  "item.examplemod.netherite_banner_shield.black": "Black Netherite Banner Shield"
}

And now your shield is fully completed and supports banner decoration!

If you are working in 1.19, make sure to change the FabricShieldLib and FAPI versions accordingly as well as changing your Midnight Lib and Mod Menu dependencies to these versions:
gradle.properties

midnightlib_version=1.0.0-fabric
mod_menu_version=4.2.0-beta.2