Admit it Stabbity, you only responded to show of the pic of your cute girlfriend
Carving from wood is probably the best way to go. Though this requires some carving skill and various tools. So long as you don't want the wood grain to show through, mistakes can be fixed with wood putty or polyester autobody filler putty (aka Bondo).
Sculpting it from clay is more forgiving, but it requires making a mold, and casting it in something like resin or a fiberglass layup. Either way it tends to be pretty pricey to make a mold and casting that size.
Carving it from extruded polystyrene foam insulation requires fewer tools than wood, but you still need to be able to carve. You harden the foam with paper mache or fiberglass+resin. It can be a bit tricky to account for the added thickness and loss of detail this gives. But on the plus side, the foam is cheap, quick and easy to cut, and mistakes aren't hard to fix.
The difference between stuff that looks professional and stuff that looks home-made (other than getting the shape in a proper proportion) is pretty much the finishing process. surfaces must be sanded smooth. With techniques like Bondo autobody filler putty, or paper mache, this sanding can take quite a long time, so most cosplayers I see seem to either skip it or not do nearly enough of it. Once smooth, porous surfaces, particularly wood must be sealed with something like sanding sealer or shellac. Finally, you need to give a good paint job. For multi-colored pieces, this generally involves liberal use of masking tape. Spraying the paint on isn't required, but it tends to be easier to get a good finish.