Well first you need to know how to write code in C# using Visual Studio, because all Portia mods (also all mods for Unity based games) are written in C# in .NET 3.5 or .NET 4.0 Then you should check the Unity Mod Manager wiki for instructions how to set up your C# project and how to start coding: https://wiki.nexusmods.com/index.php/Category:Unity_Mod_Manager You'll also need some tool to analyze DLL files, like dnSpy, to see what functions etc. are provided with a game.