To make my helmet for my black knight, I started with some metal that I salvaged from an old water heater.
I found a pattern online, I traced the pieces onto metal sheets and cut out the pieces. I used nuts and bolts to hold it together temporarily.
Once it was finished, I spray painted it black and red. The nuts and bolts were replaced with rivets. Each rivet is a nail that has been cut short. The nail head stays on the outside, and the cut part on the inside is hammered down like a real rivet. This is the same way a traditional helm would have been crafted.
I had to paint it red and black instead of just silver because my salvaged metal was already painted white and it would not come off.
I have only made three helmets total. Each one gets a little cleaner and better looking. They are not easy. It takes a lot of cutting, and hammering to shape them. The rivets are fun to put in, but they also take a lot of small hammer blows. You will be surprised how heavy these feel on your head. I think the real ones were padded with cloth and leather.
Go ahead, make a mess, have some fun.