1. Find a girl on the streets, preferably a little tipsy, but in the sweet spot where she's not so tipsy that she will fall off the handlebars in mid ride (If you are bicycling girl looking to pick up a guy hit me up, I'm always available for women on bikes.)
2. Create attraction, generate trust, convince her to hop onto your handlebars (difficult step, unless you are James Bujold)
3. Have her stand over front wheel, place both hands on the handle bars and boost her self up onto them, have your hands already in place to maximize butt touchage.
4. Begin slowly, gaining speed as you both become more comfortable (like a relationship)
5. Sing to her, especially if she is tipsy.
6. Don't jump any curbs, or hit any sweet jumps, could end in epic fail or lawsuit.
Disclaimer: There are many things that could go wrong if this is done improperly (head wounds, unwanted pregnancy if you are overly suave). We relieve ourselves on any liability. And it feels good.
P.S. Having Bull Horn handlebars make this much easier.