Looking through my old work in progress pictures, made me realize, I never posted this stage. I figure it's pretty straight forward from pattern to construction, but here's the planning stage on how I make those patterns.
After drawing my sketches on paper, and ideal on how I want to go about making each piece, I make a very very rough dummy version to show me my focal points of interest. I don't do this with every armoured piece, but helmets are notorious, and evil. The top image, shows you the axis, highest point of the helmet, width, and general shape. This dummy won't be part of the helmet at all, it's only use is for this stage. You can also see that I already made the center pattern out of paper to the left, that's ready to be cut out of cardboard for the real version.
Again, the paper pattern is a rough trial shape, in which while I construct the finish product, will get shaved down to size, added details, add darts (just like sewing) or expanded as necessary. All these lines are very cleanly cut, so they connect almost seamlessly, once painted, it's hard to tell where they are.