Well, to start off, if you want to make a decompile use Unity 2018.3.9, then download the Baldi open source project (here: https://gamebanana.com/tools/6677) After that, extract to Unity and now you can do whatever you want. (Like add rooms, change scripts, etc.) I just follow some tutorials in gamebanana or youtube to help me with my decompiles...that's all I know! (You should also credit the people who helped your decompiles/mods!)