My good friend Keksuccino, the maker of Fancy Menu for Minecraft, has put together the following tutorial. If, like me, you don’t know where to start with Mixins, then I highly recommend this.
Links talked about in the tutorial (they are also in the video description):
Mixin Examples 1 (Forge FancyMenu): github.com/Keksuccino/FancyMenu/tree/forge-1.16/src/main/java/de/keksuccino/fancymenu/mixin/client
Mixin Examples 2 (Fabric Konkrete): github.com/Keksuccino/Konkrete/tree/main/Fabric/1.16/java/de/keksuccino/konkrete/events/client/mixins