I found a cozy queen-sized flannel sheet set at the store and decided to make a queen-sized comforter cover from it. This one is easy to remove for washing and it stays put thanks to the elastic in the fitted sheet.
In summary you take a fitted sheet, cut it lengthwise, and in between the two halves you sew one half of the flat sheet also cut lengthwise. The sheet that would normally go head to foot is placed left to right across the bed.
Step 1: Step 1
First, make sure that the long side of the fitted sheet will fit over the short length of your comforter. Also check to make sure that if you were to cut the fitted sheet in half length wise and to place it over the ends of your comforter, that the space in between will be bridged by the flat sheet cut length wise. Another consideration is pattern of the print on your sheets, for example if you're using plaid, perhaps the stripes won't match well after sewing.