Cut your fabric in a rectangle shape. If you are making a cover for a 20" pillow your width should be 20 + 1 (for seam allowance)=21"and your length should be (20x2) + 3 (for your envelope "overlap")=43"
Next take your fabric and fold it over (right side in and overlap the pieces approximately 3 inches. (I didn't not hem the ends of this since it doesn't fray. If your fabric requires it you can finish the two short ends of your rectangle before proceeding).
Now sew both sides as shown (this would be your long ends of your rectangle if it weren't folded). I used a 1/2 inch seam allowance on each side. Be sure you leave your overlapping flap open.
Turn right side out and shove your pillow form in through the open flap on the back. (Or be like me and shove your dirty old pillow in there with drool stains and all.
Sorry I forgot to straighten this photo. The straightening tool (in LR 3) is seriously my FAVORITE tool ever.
Let me know if you have any questions! Otherwise, go hit up the clearance fabric section and re-cover your pillows! It literally takes 15 minutes. Or less.