Whether you're making a new cushion to fit a seat frame or upholstering an existing cushion, a box-edge fitted cushion requires accurate measurement so it fits snugly within the outline of the chair. Use foam in your desired firmness and thickness when making the cushion, and use an attractive solid or patterned fabric for the covering so it looks inviting. Use weather-resistant covers for cushions placed outdoors. Fitted cushions are easy to make and require only basic knowledge of simple sewing techniques.
Measure the length, depth and width of the seat pad to determine the dimensions of the foam. To make a fitted cushion, purchase foam that measures 1/2-inch smaller than the overall dimensions of the chair.
Place the fabric for the covering on a worktable. Brush it with your hand to remove any creases. Measure and cut two lengths of fabric for the top and bottom of the cover that measure the width of the cushion by the depth, and add 1-1/2 inches to allow for seams so the cover is fitted.
Cut two lengths of fabric that measure the cushion's length by the required depth, and add 1-1/2 inches for the seams. Both these lengths serve as the gussets of the cover --- the fabrics that hold the sides together and give the cushion depth.
Mark 5/8 inch down from the edges of the fabrics for the upper and lower sides of the cushion. Extend the mark to draw a straight line over each side.
Pin both the corners of the gusset together to form a loop, joining the right sides together. Sew the ends 5/8 inch inward from the edges to join both the ends of the fabric together.
Pin the gusset to the fabric for the top of the cushion, using straight pins. Match the corners of both pieces of fabric. Sew along the line 5/8 inch inward from the perimeter of the fabric.
Pin the lower edge of the gusset around the outer edge of the fabric for the bottom of the cushion cover. Align the outer edges of both pieces of fabric and sew the gusset to the bottom of the cushion directly along the lines spaced 5/8 inch from the edges. Leave a large 20-inch opening along one side.
Place the cushion foam in the cover through the opening. Use a slip-stitch to sew shut the 20-inch opening by hand.