How do I download the mod itself, I don't actually know how to get it onto Minecraft.

First you need to install a mod loader, either Fabric or Forge:

• https://fabricmc.net/use/installer/
• https://files.minecraftforge.net/net/minecraftforge/forge/

Then you download the AppleSkin jar file for the appropriate loader and version of Minecraft you're using (from CurseForge, Modrinth, or GitHub) and put it into the mods folder of your Minecraft instance. If you're using Fabric you'll also need to download the Fabric API jar file and put that into your mods folder, too.