vacuum forming (use the search option to find info on it) is the best way to get the shape. As for the skull design, as far as i know, the best method for that is to trace it onto the visor and then use a craft knife and a few fine grade files and clay modeling tools.
On the website instructables.com you can find instructions on how to set up a vacuum forming table. It wont be the bst, but for making a few visors, it will get the job done.
In the armor creation forum on this site, go to the pepakura section, look for the thread dedicated to Reach pep files. Get the one for the emile visor, make it, and then you have what you can vacuum form the shape onto.
