I'm aware this reply is a little late (I haven't logged in for a while).
Your best bet is to download some of the euphoria mods on the site. See what files they edit & what changes they make in-game & then compare the modded files to the vanilla files using some file compare site/service/program like diffchecker.
Make backups of the vanilla files/'.rpf' archives ('update.rpf'/'x64a>w' etc) that the files are contained within before installing the mods or making any changes & then have a little go at making your own edits, safe in the knowledge you can return to vanilla if things get messed up.
Simplest way to backup is to '.rar'/'.zip'/'.7zip' the '.rpf' in the same folder it is installed in. That way, returning to vanilla is as easy as right-click > extract here > overright file etc.
Asking the mod authors about specific things may also help your understanding too.