In this tutorial, you will learn how to register your Mixins through the resources/fabric.mod.json.

Inside your resources folder is where your fabric.mod.json should be.

Use this link to view the Fabric Example Mod's resources folder: Fabric Example Mod Resources

Inside your fabric.mod.json is where you define where Fabric should look for your mixins.json.

To register a mixin you have to tell Fabric where to look. To tell Fabric where to look you need to add elements to the mixins array inside fabric.mod.json

{
  "mixins": [
    "modid.mixins.json"
  ]
}

Providing a String "<modid>.mixins.json" inside the mixins array tells Fabric to load the mixins defined inside the file <modid>.mixins.json on both client and server.

In the previous section, you learned about registering your <modid>.mixins.json files.

We still have to define which mixins to load and where these mixins are located.

Inside your registered <modid>.mixins.json:

{
  "required": true,
  "minVersion": "0.8",
  "package": "net.fabricmc.example.mixin",
  "compatibilityLevel": "JAVA_8",
  "mixins": [],
  "client": [
    "TitleScreenMixin"
  ],
  "server": [],
  "injectors": {
    "defaultRequire": 1
  }
}

The 4 main fields you should worry about when getting started with mixins are the package field, and the mixins, client, server arrays.

The package field defines which folder (package) to find the Mixins in.

The mixins array defines which classes should be loaded on both the client and server.

The client array defines which classes should be loaded on the client.

The server array defines which classes should be loaded on the server.

Following that logic: net.fabricmc.example.mixin.TitleScreenMixin is the mixin.