Remove the old covers using a pair of scissors and some strong tearing action. Examine the cushion itself. If it is worn, discolored or smells even slightly of mildew, consider replacing them with a water-resistant foam such as EZ-Dry. Measure the old cushions and have your local sewing center cut the foam to size.
Cut three pieces of weather-resistant fabric for each cushion. Two should be the length of the seat cushion plus 1-1/2 inches by the width of the seat plus 1-1/2 inches. The additional fabric is the amount you'll take up when you make the seams, called the seam allowance. These two pieces are the top and bottom of the cushion cover. The third piece of weather-resistant cloth for each cushion should be perimeter of the cushion plus 1-1/2 inches long by the height of the cushion plus 1-1/4 inches. This is the gusset of the cover; the gusset covers the sides of the cushion.
Measure 5/8-inch in from each edge of the pieces of fabric. Using a straightedge, draw a line to indicate an inner perimeter for each fabric piece.
Pin the two ends of the gusset for the seat cushion together with the right sides facing. The right side of the fabric is the side you will see when the cover is on the cushion. Use straight pins placed a couple of inches apart perpendicular to the edges. Use a sewing machine to sew a seam 5/8-inch in from the edge.
Pin the gusset to the top of the cushion cover with the right sides facing. The outside edges of the gusset and cushion cover top should align perfectly. Use straight pins placed perpendicular to the edges of the fabric about 4 inches apart. Sew the gusset and the top of the cushion cover together using a sewing machine. Sew along the inner perimeter of the top of the cover that you indicated with a fabric marker.
Repeat Step 5 to attach the gusset to the bottom of the cushion cover, but in this case, leave a gap about 20 inches long along one side. Stuff the cushion into the cover at the opening. Hand sew the opening closed using a needle and thread. Repeat Steps 1 through 7 for the remaining cushion covers.