I had my son lay down on a piece of cardboard to trace him. which was quite difficult considering he's extremely ticklish. Then I tried to straighten up the lines and amke it symmetrical, my dad cut out the shape in a piece of plywood for me, b/c i have no saw, or plywood for that matter!
I sanded the edges, cut out some foam, and fabric like so...
TaDaa!! Once I got the hang of the electric stapler it wasn't too hard. (but be careful if using an electric stapler, their the crafty equivalent of a machine gun... Seriously!! make sure kids can't reach it if you turn your back!)
The blue one is Jersey Knit fabric which was much easier to stretch around the corners and staple down, but I suppose that depends on your level of expertise at upholstering.