So with this instructable I am going to start out with things I wish I had known ahead of time before starting the helmet.
The more hands, the better. I was using elastic bands, tape and heavey objects to temporarily hold things in place until I could secure them properly, if I had an extra set of hands to hold these things life would have been so much easier. So tip one, it's so much easier if you have someone around to help construct your cardboard base.
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.