The Helmet

Take your bust cast and throw on some clay. The helmet will take about 40 hours to sculpt for those who do not sculpt for a living. It took me three weeks to sculpt my helmet with working a full time job. Be prepared for this to take a while to sculpt.
The top of the helmet comes off the bust about .5 inch.
The ridge that circles the bottom of the helmet is about 1 1/2 in. in diameter and circle the entire piece.
The helmet is about 1.5 to 2 in. off the sides of the bust.

The Face Plate.

I recommend you not sculpting in the side vent divits. They will lock down the plastic when you vaccuum form the helmet.
The ears are separate pieces. I recommend you sculpting these after you have put a full plastic helmet together. this way the mold will be an actual representation of the finished product.
Just sculpt the ears on the finished helmet and then pull a plaster mold.
Now pull a positive mold from that and vacuum form the ears. Easier than trying to cut and bend plastic to fit your needs.
And that's the basics on Sculpting the Armor out in clay. Now all you need to do is PULL the Plaster Molds.