Box-style chair cushions are common to larger sized chairs. The cushion is typically loose fitting and may have a zipper along the back side of the cover. To sew a new cover for a box cushion is a project most homeowners can tackle with confidence. The important consideration is to choose a durable fabric for the new cover that will work with the existing chair fabric.
Remove the cushion from the chair. If the cushion has a zipper-back opening, unzip the cover and remove the cover from the cushion.
Rip the seams using a seam ripper. Take care not to tear up the old cover as you will be using the cover as a pattern for the new cover. Iron the old cover seams flat.
Wash new decorator fabric and iron it flat. Washing removes the sizing and prevents the fabric from shrinking later in a future washing. Position the old cover pieces over the new fabric and pin the pieces to the fabric. If you are matching patterns make sure all of your pieces are oriented in the direction you want.
Cut the new cover pieces using scissors.
Sew the old zipper into the new cushion back if you are reusing the zipper. Follow the sewing pattern of the old cushion for placement and positioning of the zipper.
Sew the four side pieces together. Place the back and one side, face sides together, and sew using a 1/2-inch seam allowance. Place the back and the other side, face sides together, and sew using a 1/2-inch seam allowance. Place the front and one side together and sew. Repeat with the last side and front. You will have a loop of fabric.
Position the loop of fabric sides face sides together with one of the large cover pieces. Align the corners of the large piece with the seams. Pin the seam together. Pin all four sides. Sew a seam with a 1/2-inch seam allowance around all four sides.
Position the loop of fabric face sides together with the remaining large cover piece. Align the corners to the seams. Pin all four sides if you have a zippered opening. Pin the sides and front if you do not have a zippered back. Sew a 1/2-inch seam around the pinned sides.
Turn the cover right side out through the zippered opening or along the open back seam. Fold the cushion in half. Insert the cushion into the cover. Press your hands to the front corners and align each corner with your fingers. Smooth out the cushion. Tuck the remaining cushion inside. Zipper the back closed. If you do not have a zipper, tuck the open seams to the inside 1/2-inch and hand sew the seam closed using a needle and thread.
Clip your corners before turning the cover right side out if your fabric is very thick and bulky at the corners.