First, we are going to hem the back pieces.
Take one of your 20.5″ by 13″ pieces and fold it over 1/2″ on the 20.5″ side. Iron it in place.
Next, fold over the 20.5″ side another 1/2″ and iron in place, hiding the raw edge inside.
Repeat for the other three 20.5″ by 13″ pieces.
Once you have folded all the 20.5″ edges over twice and pressed , hiding the raw edge inside, stitch the hems all the way down, with a 1/4″ seam.
Next, put your 20.5″ square piece down with the “right” side of the fabric facing up. (The “right” side is the one that has the print, and not the back (white) side)
Overlap the two back pieces. This is what will create the envelope for your pillow to be slipped in later.
Pin everything in place.
Repeat for the other three pillow covers.
Sew all the way around with a 1/4″ seam. If you have a serger you can finish the outside edges with it. If not, go all the way around with a zigzag stitch.