Nothing is a bigger pain (to me at least) than not being able to work on your project because of drying time! It's awful for an impatient person like me. If I had started my helmet and shield at the same time I could have been working on one when the other was drying. Tip two, if you're making more than one prop, might as well do them all at the same time.
A helmet too big is way easier to fix than one that is too small. If the helmet is too big you can add styrofoam or more cardboard inside to tighten it up. A helmet that is too small is a bigger problem, you'd need to start pretty much from scratch again. Tip three, when in doubt make it a bit bigger.
Now we can get started! I hope I can explain this process easy enough, it was a lot of trial and error. I googled a picture of a Spartan helmet so I could see some of the basic shapes. I started out by making the front of the mask. Take a sheet of cardboard and draw so the corrigation is going vertically, it will come in handy later on. I made it in two halves because there is a strong crease down the front and I think it could only be achieved by attaching the two halves. I drew the basic shape of the mask, and held it up to my face to match proportions and also used a ruler to measure. I cut out my shape and held it again to my face, marking with a pencil which parts needed to be tweeked. I used a boxcutter and trimmed off anything extra I needed to then traced the shape and cut out the mirror image. Tape the two halves of your mask on what ever side you choose to be the reverse side.