Use this template on GitHub or just download the code, whichever is easiest.
Rename src/TestMod.csproj, then edit mod/modinfo.json and src/Plugin.cs to customize your mod.
See the modding wiki for modinfo.json documentation.
To update your mod to work in future updates, replace PUBLIC-Assembly-CSharp.dll and HOOKS-Assembly-CSharp.dll with the equivalents found in Rain World/BepInEx/utils and Rain World/BepInEx/plugins as well as Assembly-CSharp-firstpass.dll found in Rain World/RainWorld_Data/Managed.