Envelope-back pillow covers are easy to make and easy to maintain. You don’t need to put in a zipper or make buttonholes, yet you can take the cover off to wash or change it -- easy as can be. The secret is creating an overlap between the two back pieces of the “envelope” that’s generous enough to cover the pillow form without gapping. Note: We added buttons to this example for decorative effect, but you don’t need them.
To make a pillow cover, you'll need enough fabric to cut three pieces -- one piece slightly larger than the pillow form and two pieces approximately the same size. For example, if the pillow form is 18 inches square and the fabric is 54 inches wide, you need at least two-thirds of a yard of fabric.
Measure the pillow form. For the pillow front, cut one piece of fabric those dimensions plus 1/2 inch on each side. For the pillow backs, cut two pieces 2 inches narrower than the form and 1 inch greater than the height. For example, for an 18-inch form, cut one 19 by 19 inch piece and two 16 by 19 inch pieces.
Press in 2 inches on one long edge of one pillow back. Fold in another 2 inches and press again. The raw edge will be encased in the fold. Repeat with the other pillow back.
Stitch close to the folded edge on each pressed piece, securing the fold.
Layer the pillow front and one pillow back, right sides together. On the opposite side, add the second pillow back. The right side of the second pillow back should face the wrong side of the first pillow back. The pillow backs should overlap by about 4 inches.
Pin all four sides of the pillow cover. Stitch all the way around the pillow cover, using a 1/2-inch seam allowance.
Trim off each corner at a diagonal, approximately 1/4 inch from the stitching line.
Turn the pillow cover inside out and pull the corners out square. Insert the pillow form and settle it into the corners.
Although the overlap of the envelope prevents the pillow form from showing, you can add buttonholes and buttons for the decorative effect.