1) Download a data.win file from -> https://drive.google.com/drive/folders/1WjslLoH4Z09dueO6LBugwUQUwu9pX41o?usp=sharing (if you want to install this mod with the trans Bandit mod, which sadly got removed... download this instead: https://drive.google.com/drive/folders/1_L2MHm8zbXGcOvmjPXEPA9bZ-8Dk2DY9?usp=sharing )
2) Put and replace your data.win file with the data.win file that you downloaded in -> steam\steamapps\common\Risk of Rain Returns
And thats it :3

a few things that i should probably say:
- i recommend making a backup, this shouldnt corrupt your game or anything, but just in case
- it replaces all the hats, including all the skins, this is a problem that isnt really solvable right know, but ill definitely update the mod once the official mod support gets added
- obviously this doesnt work in multiplayer, meaning you are the only one who will be able to see this
skin
- this mod will stop working after every patch, but i will update it every single time so you can redownload it

The google drive also contains the actual spritesheet of the mod,
so if the first option doesnt work for you, try doing it through undertalemodtool.