As I hate to waste any fabric, I'd suggest buying two single bed sheets and sandwich them with a lightweight batting and quilt that. Get a plain and an easy to follow pattern (ie large floral). Machine quilt it on the floral side and then you can easily check your stitches on the plain side. By following the outline of the sheet design, you don't even have to mark it. When you're done practicing, you'll still have a lightweight quilt that is useable if not beautiful!