Hey anon,
I’ll start you off with the AO3 FAQ on Archive Warnings. It came up in earlier discussions, and I think it’s a good place to start you off since you’ll probably need to use one of those.
The next guideline I suggest using is the Motion Picture Association of America’s rating scale (or the local equivalent) image linked is from the MPAA website, but since it’s at the bottom of the page I linked you directly. In terms of AO3 ratings, I tend to use the General rating for fics that I think would be G/PG if they were movies and I’ll use Teen and up for fics that are PG-13.
Personally, the difference for me between Mature and Explicit is the amount of detail and the number of times mentioned. If I generally mention something one time, I might keep the fic rated Teen, but if things are mentioned many times, even if they’re still relatively general? Then I’ll give it a Mature rating. If I say exactly what’s happening to whom, how, and what it feels like? For me, that’s explicit.
Examples of how I do things under the cut:
Oh, oh! One cannot go past the envelope distinction (by @tinsnip).
I consider it valid for scenes that are either sexual or violent in nature.