start with light tongue! guys are really sensitive there, (at-least my SO is), so don’t just dive in with your finger. use your tongue slowly and then if he’s into it, go faster with it and maybe even try and stick your tongue in a little, bit too deep tho. eventually, (if he equally wants to), barely go in with your finger, maybe just to the first knuckle, while licking underneath. keep going accordingly, and if the guy likes using fingers keep going! just do whatever feels intuitive and vibes with both of you. ALSO MAKE SURE YOUR NAILS ARE CUT of fingering, or else it’ll hurt him more than pleasure him. (I apologize if this comment makes anyone uncomfortable I’m just trying to give advice according to what my experience with this is) hope this helps!