There are several methods.
METHOD 1) (slightly hazardous) THE OVEN: Set the temp to about 200 degrees. You can either a) tape your pieces (NOT duct tape! I recommend painter's tape) to a cardboard form slightly smaller than the piece you are heating. Take the belly plate, for example. The cardboard would make the straight side of a "D" form. Heat for no more than a couple of minutes then allow to cool.
METHOD 2) HEAT GUN: You can either make a shaped form of your piece out of foam or cardboard and let the piece form to that, or heat the piece and shape it by hand. I recommend good high-temp gloves either way.
METHOD 3) BOILING: If you have a pot big enough, bring a enough water to boil that you can dunk the whole piece into in one shot. Again, with high-temp gloves, shape the piece to what you want.
With any method, there are hazards, specifically burning. BE CAREFUL!
Oh, and have fun!
BTW, as someone I know can attest to, DO NOT try to shape a hot piece of plastic directly to your body.
That's just asking for trouble.
David.