I don't know of any easy way to drain the float bowl on the carb, if that's what you're after.
For draining the tank, do as nknudsen suggested, dead simple. Make sure to stay with the bike and gas can during the operation, as you don't want the hose to accidentally pop off and drain gas and oil all over the garage floor, with the gas evaporating and the oil remaining as a slippery residue, with the stink enveloping the entire house and having your wife and daughter mad at you for a week (or two).
Or so I've heard, of course.