The funny part about this way of making the mask is: I didn't attach it to my head. At all. It actually just sits there, and quite comfortably, too, even when moving around and bending over.
I used a very light modeling structure called Model Magic. It's more expensive than baking clay and made by Crayola. You can pick it up in the kid's section of a craft store, or even kiddie projects at places like Wal Mart or even your grocery store. It comes in individual packages or tubs of four packs for about 20$. You need 3 to 4 packs to make the mask, so getting the tub is ideal. Note that once an individual pack is opened, it will begin to dry out. You have 24 hours to do what you need to do to it before it hardens like a rock.
1. Open 2 Packs of MM. Roll them into a ball and soften them up, get rid of any air bubbles. Roll them out until it is about as big and as thick as a small pizza. Take this flattened circle and rest the center of it on your head. Move all creases to the right side- it will be cut away when it dries. Make sure the left half is smooth.
2. Go about your daily business. Sit and watch TV, do homework, whatever. Just keep the MM on your head. It will harden to your head;s shape perfectly and, if done right, will not need any fasteners to stay on your head (a few hotguled bobby pins will help if you need it though)
3. When dry, follow pictures to cut the helm properly. Open the rest of the MM and add the three ridges on the mask's side, the ridge on the top, and the horn. If needed, stick a wad at the top of your ear and make an earpiece inside. MM will stick to itself, you do not need glue, but have some handy just in case. Let dry.
4, Terrorize stupid shinigami trash.
Here is a picture as to how it turned out: