Step 1: Balsa block glue-up
I made a side-view template of Iron Man's helmet by tracing a side photo of an Iron Man toy using basic drawing tools on my computer. I've included a PDF of that template just in case you are so inclined to make one of these.
Step 2: Carve basic shape, lay out details
Using the tip of a mechanical pencil, I traced and impressed the details of the mask into the balsa on either side of the block of wood. Then I drew in the details with sharpie to work as guide marks on the sides and front of the mask.
Step 3: Begin carving
If you use a utility knife in this manner it can be very useful, although EXTREMELY dangerous. You could do some serious damage to yourself or someone else if you're not careful.
Step 4: Continue carving, drawing in details as a guide
Continually adding details with a sharpie as a guide and inspecting carefully after every couple of cuts kept me from making any drastic mistakes, like removing too much material.