You ask: "How does someone go about making aluminum bulkheads for a homebuilt/scratch-built aeroplane?"

1) First you acquire/make "patterns", two identical ones for each bulkhead or rib to be manufactured, in this case we use plywood.
2) Then you buy some aluminum sheets (in this case, 2024-0 with the "-0" meaning it is NOT tempered/heat-treated yet and is very "malleable" and easily formed) and cut them to the same shape, but with some overlap.
3) Now you sandwich the aluminum in between the two forms of wood.
4) Using a rubber mallet and a few "body-working" tools, you bend the metal around the forms for a "rough" shape.
5) Strategic use of a metal "shrinker" (www.aircraft-tool.com), you can get rid of the potatochip edges and make a smooth flange.
6) Trim the excess, file smooth, and then send off to have them tempered/heat-treated (hardened) to eventually become 2024-T3 !

Voila~! One fuselage bulkhead! Now, multiply that times all the bulkheads & ribs I need and, well, you get the picture...One more bulkhead left to make - I'll do it tomorrow before I go to work, it'll be cooler then anyway.