Just do it when you're both a bit tipsy at a party or in a club or something. Then it'll be more relaxed and less intense/awkward, so if you get rejected it won't be such a big deal.
Just tell her. If her response is negative, tell her it's just a joke and that you got her good and start laughing like a lunatic even though your heart is breaking. If the response is positive, congratulations! Go rent a motel or something.
you should maybe kidnap a member of their family and then force them into marriage for their safe return, but make sure to send them a couple of fingers in the mail so they know you're serious and powerful