Remove the existing covers from your outdoor chair cushions. Unzip or unbutton them if they are removable; cut them open along a seam with scissors if they are permanently attached.
Measure the fabric pieces that compose the existing cover. Do not include the seam allowance; only measure the visible parts of the fabric.
Add 1 inch to each measurement you recorded. This creates a 1/2-inch seam allowance.
Transfer the measurements of each fabric piece including the seam allowances to the back of the new fabric with a ruler and a fabric pencil.
Cut out the new fabric pieces with scissors.
Assemble the new cushion covers. Place the top and bottom sections with their right sides together. Sew around three sides of the fabrics 1/2 inch from the edge. Leave the fourth side open. Turn the cushion cover right side out. Repeat for all of the covers.
Fold under the raw edges of the fabric 1/2 inch on the side that is open. Press to crease these seams.
Create tie closures. Insert four 6-inch sections of cord into the open seam into positions equal distances from both sides and one another so that you will be able to tie them together and create a secure closure. Add additional ties as necessary if your cushion is larger than 16 inches. Pin the ties to secure them for sewing.
Sew the pressed edges in place 1/4-inch from the edge, catching the ties in the stitching lines on both sides to secure them in place. Sew over the ties several times to ensure a strong attachment. Do not sew the opening closed; this is where you will insert the cushion.
Insert your outdoor chair cushions into your new covers and tie them closed.