Making a new cover for a chair or sofa cushion can give it a whole new look, and even make it easier to keep clean. To make a cushion cover, you'll need enough fabric to cover the top and bottom of the cushion, plus enough fabric to wrap all the way around the sides of the cushion. Make sure to choose a fabric that's machine washable if you want to be able to remove and wash the cushion cover, and prewash the fabric before you sew the cushion. To make the cushion easy to put on and take off, you'll also need an extra long zipper, such as one designed for upholstery or slipcovers, that's at least as long as the back edge of the cushion.
Cut 2 pieces of fabric, each about 1 inch longer and 1 inch wider than the cushion. These pieces will cover the top and bottom of the cushion.
Cut a strip of fabric that is 1/2 inch wider than the thickness of the cushion, and as long as one long edge plus both short edges of the top fabric. This strip will cover the sides of the cushion.
Cut a strip of fabric that is 1 1/2 inches wider than the side strip, and as long as the long edge of the top fabric. Cut this strip in half lengthwise. These narrow strips will hold the zipper on the back of the cushion cover.
Pin the 2 narrow strips right sides together, then baste them together along one long edge using a 3/4-inch seam allowance.
Pin the zipper, teeth side down, to the seam allowance. Stitch the zipper tape to the seam allowance only using a zipper foot.
Turn the fabric right side up and topstitch along both sides of the zipper, as close to the teeth as possible, using a zipper foot.
Remove the basting stitches using a seam ripper, and unzip the zipper.
Place the zipper strip and the side strip right sides together, and stitch them together along the short ends, so they form one continuous piece.
Place the long edge of the strip around the edges of the top fabric, right sides together. Pin the strip to the edge of the fabric, then stitch them together using a1/4-inch seam allowance.
Pin the other long edge of the strip to the bottom fabric, right sides together. Stitch them together.
Turn the cushion cover right side out and insert the cushion into the cover through the opening in the back. Zip the zipper.
If you can find a zipper long enough, you may want to use a zipper that's a bit longer than the back of the cushion, and adjust the length of the fabric strips to match, so that the zipper wraps around the corners. This will make the opening wider, so it's even easier to insert the cushion.
Use caution when working with scissors, pins and sewing machines.