Before installing any mods, first locate your Rain World folder. This can be located easily from your Steam install directory. From the Steam directory, go to "steamapps/common/Rain World". Then backup your important files, especially your save files! Saves are located in the UserData folder; make a copy of them somewhere.
Partiality Mods:Any number of these types of mods can be used together at the same time, using the Partiality Launcher.
Standalone Mods:Standalone mods are mods that can only be used one at a time. Certain mods come with a README file. Always be sure to check those for special install instructions.
Navigate to the RainWorld_Data/Managed folder, and replace ‘Assembly-CSharp.dll’ with your modded .dll. If you have Partiality installed, then you will also need to replace the 'Assembly-CSharp.dll' file in the RainWorld_Data/Managed_Backup folder.
That’s it, the mod is now installed! To uninstall the mod and revert the game to its normal state, right click the game in Steam, and use "Verify Integrity of Game Cache". Then restore the save files you backed up. If you have Partiality installed, you will also need to delete the Managed_Backup folder to complete the uninstall.
Essential Download Links:
Dev Tools are in-game debug and editing tools that can be used to make basic modifications to the pre-existing maps. They are a good starting point for new modders.
The Official Level Editor is used to make new maps that can be added into the game. It is the actual editor that the developers created and used to make all of the maps in the game, so it is very powerful! However, it is also quite unyieldy, and requires patience, experimentation, and tenacity to learn.
dnSpy is an assembly decompiler tool for advanced modders with programming experience who intend to make changes to the game's code.