How to promote a fic?

I am around 75% done with a crossover one shot in 2-3 REALLY dead fandoms (if they were ever alive). I know that writing fanfic isn’t about how many people see it and all of that, but I figure that learning how to promote fics is also going to be helpful for my more long fics in bigger fandoms.

I do not use much social media. Reddit and AO3 is about as far as I like to go, but I do have tumblr ig. Any tips?