So if you got this game to start working on mods for it, and you have absolutely no idea where to go, this guide is for you!
It's not going to give you a step-by-step on how to make your own, but it'll get you in the right places to start learning.

Honestly, the best place to get help and take your first steps is to join the official Discord server:
https://discord.com/invite/thekarters
This isn't me getting clout for getting more members into the server, I just really like the few people I've so far talked to here, and pretty much everything you can find for modding can be found here, including some tools for non-conventional racers (i.e: the skater and biker characters)

Besides the Discord server, there's also the GitHub page for the official Workshop starter project, which gives you everything you need to make pretty much anything you could want for this game,
https://github.com/PixelEdgeGames/TheKarters_WorkshopProjectRelease/wiki
It tells you all you need to download to get everything up and started.

As you can see here, it also links to the Discord server, just another reason to go check it out. Again: Not Sponsored

Ok, I added this as a joke, y'know like how YouTube use to have star ratings, but then I realized that Steam guides actually have star ratings, so uh yeah