I found this great tutorial on the Floating World blog, took a few measurements and drew up a pattern. The fabrics were purchased from Lindcraft but I couldn't find cotton batting there so I found this great eBay seller and bought it there (also got some toy rattle and squeaker inserts for future knitted toys - I've never seen these anywhere before!). I even pre-washed the fabric. It's not something I normally do (although I know I should) but after a recent shinkage disaster I didn't want to take my chances. Unlike the tutorial, I didn't use binding to finish the edges, but rather sewed it together with right sides facing, turned it in the right way and then top stitched it.
The tutorial instructs you to make friends with the buttonhole function on your sewing machine. Unfortunately my buttonhole function and I are no longer on speaking terms. After a couple of hours of screaming and swearing I got there in the end, but not before chewing up all of the green cotton I was using, so 12 out of 14 buttonholes are black.
After the stress of it all, I am very happy with how they turned out. Mind you, next time I might just buy them!