Here's how I made my fabbo leather-covered bicycle-aviator's helmet which I wore to the local Tweed Ride today.
First, I got a nice sharp pair of scissors and an old leather jacket. The leather was quite thin and supple, almost stretchy, and my sewing machine had no trouble with it.
First I cut a strip as wide as I could while keeping it flattish across the top of the helmet, from the front of the helmet right over to the back. I kept it a good 5cm longer at front and back to allow me flexibility and some breathing space when adding the side panels.
The next bits involved a bit of guesswork. First I guessed at a triangle-type shape, with a slight curve to one side, and cut two mirrored pieces. This is the bit to right of the photo below, just near the handle of the scissors. I attached each triangle piece with a seam running along the middle panel, so at that point there was a long panel with two triangles attached on either side. Then I draped it over the helmet and laid a piece of leather over the top, and traced an approximation of the side panel pieces, and extended the shape down to a long ear flap piece. At this point I left the ear flaps large and mostly unformed so I could adjust them later. I attached these two side panels to the existing pieces, so only 2 more pieces left to add.
I made a bit of a guess at the final two pieces, which were little triangles almost the same as the front. I made the point of the seams where three pieces met (at 4 spots on the top of the helmet) to co-incide with the 4 points on the helmet which were most prominent, or where the curve changed the most.
Before I glued the basic form to the helmet, I made a couple of long thin rectangles of leather and made a loop and buckle which I attached to the back of the helmet. This was mostly for design and aesthetic purposes - on a real aviator helmet it would adjust the tightness across the back, but I added it both to look good, and to give the goggles' elastic something to tuck under when they were put over the finished piece. The buckles were also rescued from an old pair of trousers which I picked up at an opshop and only wore once, so they had a battered and authentic appearance :) Unfortunately they say "kathmandu" on them, if you look closely at the pics in the next post ;) but you have to be pretty close!