How to knit a case for a DS Lite using two straight needles?

I’m a beginner first off. I tried using circular needles to knit a sock or case or whatever you want to call it for my boyfriend’s DS Lite and since it was so narrow I couldn’t get the stitches to stretch enough out in order to knit in the round. I also tried the magic loop method but became frustrated. So now I want to knit a “rectangle” and then stitch up the side and bottom in order to make it into a case. I tried making the seams before on my own and it was a disaster. Any tips please? I just want to make a slip on case with a tab top that I can use to Velcro it shut.

In case you guys don’t know what a DS Lite is:


Knit your square to the size of the DS. You an even put 2-3 inches of ribbing at the top like the pattern you had been trying out. You don’t want the size much bigger then it will take to wrap around the DS; you’ll only need one stitch on each end for the seam.
look down this page of videos to the one on Mattress Stitch - that’s how you want to do your seams.