You start with an idea, and a willingness to be independent. Asking for help if you've run into a problem is one thing, but asking others to do things like actually doing the coding and assets for you is certainly frowned upon.
You then hop over to the CSTSF Modding Discord and look in their #modding-tutorials channel. As far as I know, those resources are the most up to date and focus more on Booster's Lab rather than Cave Editor, which is what the basic editor's guide on the Tribute Site focuses on. That also seems to be the place where Cave Story modders are most active, but...I could be wrong.