Measure the pillow. Write down the length and width of the face of the pillow. Add 1 inch to the measurements. Measure the length and width of the left and bottom sides of the pillow, and write the figures on the piece of paper as well. If the pillow is square, you will only need to get the dimensions for one of the pillow's sides. Add 1 inch to the second set of measurements.
Cut the fabric according to the Step 1 calculations. You will need two equal pieces of fabric for the front and back of the pillow cover and four narrower pieces for the sides.
Lay the cut fabric, wrong side up, on a table. Place the two largest pieces on top of one another. Place each of the four pieces remaining fabric around the respective sides of the large one, matching their lengths. If the pillow is not square, this will prevent you from accidentally sewing pieces together that do not match in length.
Pin the narrow lengths of fabric that make up the sides of the pillow cover around one of the large pieces of fabric, with the right sides facing each other. Make sure the corners of the fabric line up correctly.
Sew the pinned fabric together using a 1/2-inch seam allowance. Remove the pins as you sew.
Fold the non-sewn length of one of the longest sides of the cushion, with the wrong side facing in 1/2 inch. Pin the fabric along the fold.
Fold the second large piece of fabric, with the wrong side facing in 1/2 inch and pin. The pinned side of the large fabric should be equal in length as the piece of fabric you pinned in Step 6.
Center the zipper along the pinned edge of fabric in Step 6. The front of the zipper should be on the same side as the right side of the fabric. Pin the zipper in place.
Lay the pinned side of the large piece of fabric from Step 7 next to the piece of fabric that has the attached zipper. Make sure the pieces of fabric lined up evenly; pin the free part zipper to the large piece of fabric, at the fold.
Sew the zipper in place, using a 1/4-inch seam allowance from the folded side of the fabric.
Remove the pins from the folded edges of fabric and pin the two pieces of fabric together, with the wrong sides facing each other, on either side of the zipper.
Sew the pinned edges together, using a 1/2-inch seam allowance. Only sew the fabric, not over the zipper.
Open the zipper.
Pin the second large piece of fabric to the three remaining narrow pieces of fabric to form a cube-like shape. All the wrong sides of the pieces of fabric should face out.
Sew the pinned edges together, using a 1/2-inch seam allowance. Remove the pins as you sew.
Sew the open corner edges of the patio cushion cover together, using a 1/2-seam allowance.
Turn the patio cushion cover right-side-out through the opening at the zipper.