Tell her casually after your first noticable stutter. Just adress it mid sentence like : "O yeah, I didn't tell you, I stutter sometimes , anyway, a duck and a lawyer walk into a bar,....". Be confident about it. Act like it's no big deal CaUsE IT IS NOt ! Don't try to hide it, BE your self ! Being confident about a flaw makes you look, strong, attractive, but also that you are a nice human being with flaws, like everybody els. If you can create that atmosphere, you are the man. You Got This !! TPeople mostly only care if you care. Even those little gniffles that sometimes happen behind your back, be like teflon, because fuck them.