How flat chested? If she's like an A cup, that would be easy. What I like to do to my girlfriend who is a 34B is I approach her actual breasts first, farthest away from the nipple, by gong under her shirt, and putting my thumb and forefinger under her bra, (the underwire,) and pushing her boobs up 2-4 times, but pulsing. Then I run my hands toward the back of the bra, and unhook it. Try to do this with warm hands, and this is all when you're kissing by the way. Then I go back to the boobs, and start rubbing, going slowly toward the nipple, and then just tickling the nipple with my thumb, while using the thumb and middle finger to hold the breasts up. You should be good with this dude.