Step 2: The frame
Cut stripes of 4cm width out of regular cardboard and glue two of them together for double thickness. Glue one of these double-sripes together to make a ring - the base ring - and a shorter doble-stripe on the inside of he base ring for the helmets upper brace. Then glue another brace on top of it on the outside, to make it even to the ring. This frame will be on the outside of he helmet so you want to do things very accurate.
I have mesured the required head circumference of 53 cm. When i built the first frame, it was way too small. The helmet will be very circular, but a head usually is not. At my third attempt I used 60 cm stripes for the ring, which turned out to be a little too wide, but fits fine with some pad inside the helmet.