It's possible to really do a cardboard helmet
well, but I chose to go a faster and easier route. I measured around my head at the largest point (about the forehead level) and cut a strip of cardboard that long plus one inch for ease and one inch for overlap. The height I estimated by holding a ruler up to my head in a mirror.
I cut the helmet piece at the edge of an existing edge of the original box, and went over about an inch. I made a lot of vertical scores, after which I was able to easily roll up the cardboard. I sliced small V's out of the part past the existing foldline, so they wouldn't interfere when the cardboard was curved into a circle.
I drew an oval with a circumference of the same length as the head measurement, which in my case was 24". Most heads are oval shaped to some degree, you nearly never get a very circular one.
I wrapped the bent strip around the oval and marked where the overlap came to, and hot glued it together into a tube. I pushed the crown to the top where the V-shaped flaps were, and glued one flap at each end to keep it in place. Then I glued the rest of the flaps, one at a time. I found it was easier to put the glue on the flaps than on the crown piece.
Once that was done I decided to reinforce the crown with a second piece of cardboard on the top. I simply traced the outline, cut another piece, and glued that on.