My current main project is a Yamaha XS400, I'm turning into a cafe racer, or as close as I can get to one. I'm not putting any particular emphasis on period correctness (considering its an 80s Japanese bike meant to emulate 50s British bikes), my focus will be on looks and performance.
A key part of a cafe racer is the seat, there are many different styles, I'm going with a classic butt-stop seat. It has a raised hump at the back to support you under acceleration. I started by making a pattern off of my gas tank, so that the sizing and shape would be at least vaguely similar. I placed a sheet a construction paper against the tank then traced the body lines and cut around them.
I went through several versions of the pattern, adapting them by eye and placing them on the bike until I got the shape I wanted. )If you do this, you'll have to visualize what the flat paper will look like once its been shaped. Its not always easy.) Then I transfer the pattern to my sheet of aluminum.
And cut it out with tin snips.
Remember to de-bur the edges of cut sheet metal, they can be extremely dangerous so just running a file over the edges can save you countless adhesive medical strips.
This is also a good time to correct any errors in cutting you may have made:
You can see here that I left a hard angled corner at one end.
But the file makes short work of the corner.
My next step was to add some strength to the legs, they were very floppy and hard to manage and I was concerned they might get damaged if they were to flexible. So I folded the bottom of each side into a lip by hammering it over a piece of angle iron.
The metal will probably warp a little because you're stretching it every time the hammer strikes, but that can be fixed easily with hand adjustments.