Oh Ben...when is this formal?with things like this, its always best to get to her WAY in advance...the way you said you have to do it tomorrow makes me think that it is soon...dont leave it late again...or if you haven'tleft it late...um,fair play.

If she turns you down, dont let it get weird!
Things like this only ever get weird if you let them get weird...the key is to not let it get weird if she turns you down...I'm not saying you shouldn't be disappointed...just dont cry in front of her or anything...its not even a problem if you look a bit disappointed...but dont let it change things after that...the next time you see her, dont be shy or embarrased...

anyway, on to the true response of when she says "Yes"
Do what you want after she says "yes"...like was said before, you've already won...play it cool tho, dont do a victory dance or anything.
tis probably best to make meeting arrangements, and possibly just chat for a bit.

do let us know how it goes dude...Good luck brother