I don't know if there's many changes in FL Studio compared to just FruityLoops 3.x...if not, cut your sample into an even set of bars, usually 2 or 4, using your .wav editor. Load it into FL, play it on the first note (I would set FL to a 64 pattern) and then just adjust the tempo until it fits...at least that's how I used to do it some years ago when I used FruityLoops.