Measure the length, width and depth of the cushion and record these measurements.
Cut the top and bottom cushion fabric equal to the width and length of the measurements taken plus 1-1/2 inches. For example, if the cushion is 20-by-24 inches, cut the fabric 21-1/2-by-25-1/2 inches.
Cut the boxing fabric strip width equal to the depth of the cushion, plus 1-1/2 inches. Cut the length equal to the sum of the width of the cushion plus twice the length less 10 inches, plus 1-1/2 inches. This will make a strip long enough to wrap around the front of the cushion, toward the back on each side, terminating 5 inches before the back corners. The zipper section will run across the back, wrap around the back corners and continue toward the front for 5 inches. Having the zipper wrap around the corners eases the insertion of the cushion into the cover.
Cut the two zipper strips equal to the width of the cushion, plus 10 inches, plus 1-1/2 inches. Fold these strips in half along the length and press the fold.
Lay one folded strip, the fold to the right, on the zipper, aligning the center of the zipper to the pressed edge. Sew the strip and zipper together with a seam 1/8 inch to the left of the folded edge. Sew the second strip to the remaining side of the zipper.
Measure the length of the zipper strip and mark the center with a pin on each long edge.
Match one end of the zipper strip to one end of the boxing strip, right sides together, and sew with a 3/4-inch seam, opening the zipper as necessary to avoid sewing over the zipper pull.
Sew the zipper strip and boxing strip together at the zipper end, backstitching over the zipper. This creates a continuous boxing strip.
Mark the center back of the cushion cover on the cut edge.
With right sides together, align the center back of the cover with the center pin of the zipper strip. Pin the strip to the cover all around the perimeter of the cushion cover. Sew the boxing strip to the cover, using a 3/4-inch seam allowance.
Open the zipper 10 inches to allow access to turn the cover when completed.
Align the center back of the remaining cover piece with the center pin of the remaining edge of the zipper strip and pin the strip and cover together, all around the perimeter of the cushion.
Sew the strip to the cover, using a 3/4- inch seam allowance.
Finish the seams with a zig-zag or serger, if required.
Turn the cushion right side out and insert the cushion form.