Measure the seat of your rocking chair -- you need this measurement to get a cushion of the appropriate size. With this measurement, visit a craft store or home furnishings retailer and buy a plain seat cushion that fits your rocker -- since rockers are relatively uniform in size, this is not difficult. You may also substitute a thick foam pad for an actual cushion since it will be covered by the end of the project.
Purchase several yards of fabric. While upholstery fabric is more durable, feel free to use any cotton or cotton blend fabric for this project. You need approximately 1/2-yard more fabric than twice the dimensions of your cushion.
Cut the fabric so that you have two squares, each of which is several inches larger than the cushion on each side. Place the squares one on top of each other so that the outside of the fabric faces in then pin them together on three sides. Slip the cushion into the opening to make sure it fits.
Use a sewing machine to stitch the two squares together along three sides, leaving one of them open to make a cushion cover similar to a pillowcase. Turn the fabric inside-out so that the stitching is hidden inside the case.
Cut two strips of fabric out of your extra material. Each strip should be at least 4 inches long and 1 inch wide. Iron them so that they are flat and straight then do a straight-stitch 1/4-inch in from the edge along both sides of each strip.
Pin the center of one of the corner strips along the opening of your cushion case. Use a small straight stitch to hold it in place then repeat the process with the other strip in the adjacent corner.
Place your cushion on the seat of the rocker with the opening facing the back of the chair. Gently secure it in place by tying the two strips around the vertical slats of the chair.