In this example, 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", 
    {
      "environment": "client",
      "config": "modid.client.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.

Providing an Object allows for deeper customization.

Taking a closer look at the Object from lines 4 to 7:

{
  "environment": "client", 
  "config": "modid.client.mixins.json"
}

The environment field is set to 'client'. Which lets Fabric know we want these mixins to only be loaded on the client.

The config field should point to the file where all the mixins for that environment are defined.

In the previous section, you learned about registering your client/server <modid>.mixins.json and environment specific <modid>.<environment>.mixins.json files.

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

Inside your registered <modid>.<environment>.mixins.json:

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

The 2 main fields you should worry about when getting started with mixins are the package field and client array.

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

The client array defines which classes should be loaded.

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