So you want to mod Risk of Rain 2?
Modding Risk of Rain 2 can be really demanding. Depending on what a solo developer is doing, they might need to know the entire game development stack (coding, modeling, sound design, etc). At the bare minimum, at least programming knowledge is required, and custom models and sound effects add to a mod's quality. Oftentimes people work as teams or commission others for skill sets, they're missing, but it is possible to do everything solo.
This page is meant to be a learning reference for any skills you might feel weak in or want to learn so you can mod the game effectively. You don't need to do it all to make good mods!
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Programming
This free video course from freecodecamp will explain to you the basics of c# programming.
If reading is more your style, you can find a written guide you follow along with from microsoft here.
Unity
Modeling
You can find the iconic blender donut tutorial here.
Other channels you can learn from include Grant Abbitt and Ian McGlasham.