I used the wizardofflight pattern from The Dented Helmet forums, scaled at 98% for the pattern. I had originally tried to use for-sale signs and bondo, but decided to redo it in sturdier Sintra (PVC Foamboard)
The top two shots are the sintra pre-shaping attached to a child's bike helmet (I'm small and need a tinier helm). I initially rounded it over a plastic bucket with the heat gun and riveted the shield to the helmet dome, since glue wasn't sturdy enough. I also had nipped a section and reshaped the front of the bike helmet so the face would be more straight rather than protrude out at an angle.
The bottom two are post-shaping and cheeks put in. The shield followed the oval curve of the helmet, but the bottom needed to be circular. I found if I heated small sections with a heat gun and squeezed it round from either side of the helmet, it forced it round without distorting it. I inserted a craft ring with superglue and hot glue, as well as the cheeks. The eyebrow shape is my own design. I've since added the additional layer around the t-visor.
Next up, keyholes and bondo. And sanding. Lots and lots of sanding.