Measure the chair bottom and back. Record your measurements. If your seat is not square or rectangular, make a paper pattern of its shape. Add 1/2 inch to all sides for the seam allowances.
Cut two pieces of the outdoor fabric you have chosen, according to the dimensions, for each cushion -- a top and a bottom. Cut four strips of the fabric 5 inches wide, two the length of the cushion and two the width of the cushion. These pieces will make the welt or gusset of the cushion.
Sew gusset/welt strips together to form a ring. Leave 1/2 inch free at the top and bottom of each gusset seam -- you should have 4 inches of stitching in the 5-inch welt. These seams will be at the corners of your cushion top and bottom.
Pin the welt ring to the top piece of cushion, right sides together and aligning the welt seams to the corners. Pin in place and stitch. Stitch again for reinforcement.
Pin the bottom cushion piece to the welt ring, right sides together, aligning seams with corners. Stitch around three sides, leaving the back side open. Reinforce by stitching again. Turn the right sides out.
Install a zipper in back opening. Mark the foam to the dimensions measured. Cut the foam to size and insert through the zipper opening. Zip it up.