If I can just bump into this, I think at the moment to make helmets like that absolutely perfect is a bunch of trial and error. The templates you find here are accurate enough to get started on recreating a design, but the helmets in game are mapped just a bit differently which means something that looks good on the template isn't necessarily going to look the same in game. [Edit] I should point out that this is because the models are pulled from earlier versions of the game, and as far as I know there's no way (yet) to export the models out of the new game.
A few pointers;
- In photoshop you can generate a wireframe-layer that gives you an idea how certain parts of the texture interconnect;
- The fastest way to get correct placement is to position it the way you want it as a new layer on top of the 3d model view, then right-click the layer and choose 'merge down';
- If you're new to photoshop, check out some tutorials on masking and using the pen-tool. The pen is essential in making cool designs, IMO, and masking makes designing so much easier.
And last but not least... Just get started. Think of an idea an create it until you're satisfied.