Muffing involves stimulating the inguinal canal, which is the body cavity where the testes descend from. It usually takes some practice to figure out with a partner. Communicate, use lube, and go slow until you find a spot where you can clearly put in a finger tip- start by massaging using just a fingertip and work up over time. The ultimate guide to muffing comes from a zine by Mira Bellwether. There’s also free, but less comprehensive tips on muffing (as well as other sex tips from and for trans people). It’s best to figure this out on yourself first so that you can communicate with a partner what it should feel like. Go muff yourself!