As crazy as it sounds, start with the barrel. This enables you to then get the slides' width from the front view and to get the shroud that wraps over the barrel for the foresight. Once you have the curvature right for the slide body, you then need to switch to top view to trim the curved surfaces at the front of the slide where it blends toward the muzzle of the barrel. I modelled a concept Beretta 9mm just for fun about a year or so ago and the slide is definitely the hardest/trickiest part.

Martin
(TpwUK)