easy, friend, i will tell you exactly what to do.
first, start off by inviting her to an expensive dinner to eat some dish that she really likes, tell her its the best in the entire world and the chef is about to die from some illness, so this is the last chance she has to enjoy it.
second, make sure that the restaurant has a sky view.. that should probably be in the first step actually, oh well. why? because at exactly 9:15pm ...
third, ...you would of planned to have a massive fireworks display take place, catching her by surprise. at the end of the display the fireworks will spell out something like "will you be my girlfriend, [insert name]"
fourth, while she tries to figure out if its her name that the request is directed to, you confirm it by surprising her with a diamond ring and a romantic chant... something like "baby, you are the most beautiful girl in the world, no matter what they say"
bang

shes yours forever. oh, and make sure she doesnt have a boyfriend first.
good luck!