the procedure: if it's done with anesthesia, no pain; if done without anesthesia or it doesn't catch on, the pain will be on the verge of unbearable; only other thing to add is that even with anesthesia you'll feel pressure while he separates the skin, on the chest and arm this pressure isn't much of a bother, it seems like someone is sitting on you without making it hard to breathe, but on the head it can be pretty discomfortable, nothing out of hand, but you'll want it to be over

after: the healing is pretty chill, not much pain or anything, granted you don't try to lift weights or anything in the likes, or sleep on it, or get punched near it... just takes a few days off and it'll be 100% before you know it

Your position will depend somewhat on the size of your breasts. They can distort image/size of area when you're lying down due to pushing up toward your neck. It is possible that you'll be somewhat inclined but not fully sitting, though every artist and procedure can be different. I was on a bit of an incline for my chest piece, and had I not been, I probably would've suffocated and ended up with a very, very odd looking piece. Haha.