First I like to start with a pattern, foam is fine to use as a template, especially when it doesn't require an exact copy, but you're just working out a shape. It helps to figure out mounting and clearance issues with the bike before banging out any metal too.
Depending on your style, you can start forming any part of the tank first, I usually like to start the sides on manx types first, because the top is mostly flat and a lot simpler. I like to be able to put each side next to each other to guarantee symmetry.
Since the pics were just in progress pics, I'll detail more of the actual forming process in the next few days' posts.