I'm gonna warn you now, if you don't have any former modding/coding experience, this will take months or years. There's a lot of different parts about modding that sometimes require different skills for each, and things like particles, tutorials, and shaders especially can be pretty difficult to understand even with practice
Anyway as far as starting modding goes, the best first step is to make a mod that clones a vanilla part. It sounds a little boring and it kinda is, but getting a working mod with a part added is a solid step for beginners. Once you have that working, start tweaking bits here and there one at a time, seeing what works and what doesn't. Remember that the more you change without launching the game, the more things you have to debug if it crashes or the part stops working. Once you feel confident in editing that part, try making a different one, or adding something new onto your existing part to make it more unique. Keep practicing/messing around and you should be able to start making custom parts of your own without issue. More complicated parts are of course harder, so again take it one step at a time.
Lastly don't get discouraged by the game crashing. Cosmoteer is specifically designed to crash if basically anything goes wrong, that way almost all issues get noticed immediately and the game (and usually mods) are relatively issue-free. Every modder, no matter how experienced, probably crashes the game more often than not. Also when Cosmoteer crashes it shows something called a stacktrace, which basically tells you exactly what function threw an error, that function's parent function, that function's parent function, etc, all the way to the main thread. It looks really daunting (since it easily takes up your entire monitor in text), but the information on how to fix the crash is usually clearly outlined in the first few lines.
If you have any issues you can't figure out, feel free to ask the forum about it, or join the discord (if you're old enough for the TOS) where a lot of the community's action occurs. When you're asking for help, always provide the crash log and if applicable the code in question.
Hope I didn't just scare you off lol, happy modding!