Fabric Wiki

GDLauncher is a modern, simple, and powerful custom Minecraft launcher, that allows you to create and manage multiple separate instances of Minecraft without conflicts. The idea is similar to MultiMC but arguably has a more modern and user-friendly GUI, as well as being more beginner-friendly overall. Just like MultiMC, GDLauncher is open-source.
GDLauncher also has integration with CurseForge, which allows you to install mods directly from GDLauncher, without having to navigate to CurseForge and downloading the jar files. For some, this will be a very handy feature and worth switching to from MultiMC, and should help beginners find the correct version of the mods they are trying to install easier.

To download and install GDLauncher, follow this link, or go to their GitHub releases to download the portable edition. For this tutorial's sake, just download the installer from GDLauncher and run it, but if you want to customize the install location of GDLauncher, I recommend downloading the portable edition and extracting the files to the directory of your choice.

If you're using the installer, just follow along with the installer until GDLauncher is installed.

GDLauncher comes with a version of Java 8 pre-installed, but in case it becomes out-of-date in the near future, make sure to update Java: Install/Verify Java (Windows)

On your first launch of GDLauncher, you may be greeted with a prompt asking you to set up Java. If you are a beginner and is eager to jump directly into Minecraft, I recommend clicking “Automatic setup”, as you can always change it down the road. If you would like to mess around with Java a bit, then pick “Manual setup”.

If you clicked “Automatic Setup”, you may skip “STEP 2” and jump directly to STEP 3.

For those who clicked “Manual setup”, follow along. A prompt asking you for a custom Java executable should appear.

Browse to your Java's directory, and select “java.exe” (not “javaw.exe”, since GDLauncher automatically looks for “javaw.exe”)

Select “Continue with custom java”, and you should be good to go.

In order to get started with setting up an instance for Minecraft, GDLauncher will ask you for your Mojang account and password. Enter your Mojang credentials and select “Sign In”.

To set up an instance in GDLauncher, locate the “Create” button in the bottom-left.

GDLauncher will ask for a version.

Select “Fabric”, then select either “Releases” or “Snapshots”, depending on what you're doing. It is recommended to download the latest version of Fabric to avoid any bugs or compatibility issues. Finally, select the version of Fabric you would like to install.
Enter a name for your instance.

Your instance should now be created and installing the version of Fabric you specified.

“The Fabric API is the core library for the most common hooks and intercompatibility measures utilized by mods using the Fabric toolchain.” - comp500, wise man

As stated before, GDLauncher has tight integration with CurseForge and therefore allows you to browse CurseForge mods directly from GDLauncher.
To install Fabric API from GDLauncher, find your instance, and right-click on it. A context menu should show up, and click “Manage”.

Inside of the “Instance Manager”, navigate over to the “Mods” tab, and select “Add Mod”.

In the window that pops up, type in “Fabric API”, find Fabric API, and hover over the mod. You should see an “Install” button fade in, so click that to install.

The correct/latest version of Fabric API should now be installed.
Much of the same is said for installing mods with GDLauncher. If you would like to see the list of mods that you have currently installed, install the Mod Menu (Fabric) mod. In the same window, search for “Mod Menu”, and install the Mod Menu mod.

You shouldn't have to worry about the mod version yourself since GDLauncher will only show mods that are compatible with your instance's version. However, if you would like to check, you can click on the mod you are trying to install, and it should give you a more detailed overview of what the mod is, and a description of the mod.
These are some examples: