In your item class, override appendTooltip like so (see lang for how to translate the tooltip).

For versions 1.18.2 and before:

@Override
public void appendTooltip(ItemStack itemStack, World world, List<Text> tooltip, TooltipContext tooltipContext) {
 
    // default white text
    tooltip.add(new TranslatableText("item.tutorial.custom_item.tooltip"));
 
    // formatted red text
    tooltip.add(new TranslatableText("item.tutorial.custom_item.tooltip").formatted(Formatting.RED));
}

For versions since 1.19:

@Override
public void appendTooltip(ItemStack itemStack, World world, List<Text> tooltip, TooltipContext tooltipContext) {
    tooltip.add(Text.translatable("item.tutorial.custom_item.tooltip"));
}

For versions since 1.20.5:

@Override
public void appendTooltip(ItemStack itemStack, TooltipContext context, List<Text> tooltip, TooltipType type) {
    tooltip.add(Text.translatable("item.tutorial.custom_item.tooltip"));
}

You can also append your tooltip in the block class by overriding a similar method:

For versions below 1.20.5:

@Override
public void appendTooltip(ItemStack itemStack, BlockView world, List<Text> tooltip, TooltipContext tooltipContext) {
    // for versions 1.18.2 and before
    tooltip.add(new TranslatableText("block.tutorial.custom_block.tooltip"));
 
    // for versions since 1.19
    tooltip.add(Text.translatable("block.tutorial.custom_block.tooltip"));
}

For versions since 1.20.5:

@Override
public void appendTooltip(ItemStack itemStack, Item.TooltipContext context, List<Text> tooltip, TooltipType options) {
    tooltip.add(Text.translatable("block.tutorial.custom_block.tooltip"));
}