A secondary fermentation is best for a fruit beer. Do your beer as you normally would. After primary fermentation has completed, add some cherries (preferably a puree) to your secondary fermenter and rack the beer on top of the cherries. I would recomend a blow off tube for the secondary as well. As far as how much cherry, I'm not sure. If i had to guess, 3-4 #'s of a cherry puree should work just fine.
Or you could take an easier route and add cherry extract at bottling/kegging, but i always prefer the real thing.