You shouldn't need a water pan, and for the first one I would keep it simple. Trim the hard fat, leave the fat cap intact (maybe trim to 1/4"), rub it, cook it indirect at 260-270, fat cap down. It will take 60-75 minutes per pound as a rough estimate.
I start checking for done at around 190 internal and go by feel. Probe the flat and the point will take care of itself. Cook it until your probe in the thickest part of the flat goes in with little to no resistance.
Once it probes tender wrap it in a couple of layers of heavy duty foil and let it rest in a dry, preheated cooler insulated with some clean towels or newspaper for a couple of hours.