I decided making a cover for my Nintendo DS would be easy. The instructor I had at my basic sewing class warned us new people to never attempt sewing without a pattern... but it came out ok!
My first attempt I used crappy cotton cloth, added some light interfacing, and used fleece inside. I estimated the sizes, cut the fabric, and sewed it all together. It looks like this:
The seams don't match up, but I could live with that flaw -- what I couldn't stand was how awful the fabric quality was. It was a fat quarter from JoAnns, and I'm never buying them again.
My second try, ended up like this:
This time I used home dec fabric with fleece as liner. It's much much better and fits my DS perfectly. (It does fit entirely inside the sleeve)