Take off the seat cushion of your easy chair or take one seat cushion from one end of your couch. Place the storage ottoman in front of the chair or in front of the couch on the end without the cushion. If you use the couch, the ottoman and couch will form an L-shape. The height of the ottoman must be the same as the chair or couch seat without the cushion.
Measure from the front edge of the storage ottoman to the back of the chair or couch seat. This is your length measurement. Measure the width of the ottoman where it meets the chair or couch. This is your width measurement.
Draw a rectangle to your length and width measurements with a permanent marker on a foam slab that is 4 or 5 inches thick. Use a regular electric kitchen knife to trim the foam to size using the markings. This is your cushion.
Choose fabric for the cushion cover that is durable, goes with your furniture's existing upholstery, and does not stretch. Cut two equal rectangles of this fabric for the top and bottom of the cushion cover. The rectangle dimensions should be your length and width measurements plus 1 inch for seam allowance.
Measure the thickness of your foam slab and add 1 inch for seam allowance. Measure all the way around the sides of the foam slab and add 1 inch seam allowance. Cut a long, thin rectangle of the fabric using these two measurements. This is the side piece of the cushion cover.
Start in the middle of one side of the top piece of fabric. With right sides together, pin one edge of the side piece to all four edges of the top piece. The side piece should lie toward the middle of the large rectangle. Sew them together with a ½-inch seam.
Pin three sides of the bottom fabric piece to the free edges of the side piece, keeping right sides together and matching the corners of the top piece. It should form the inside-out cushion cover. Sew the three pinned sides of the bottom piece to the side piece with a ½-inch seam. Leave the unpinned short side open. Remove all the pins and turn the cushion cover right-side out.
Slide the foam slab into the cover. Turn the raw edges of the short side of the cover you didn’t sew to the inside 1/2 inch. Stitch the opening closed by hand. Place your finished covered cushion over the chair or the end of the couch so that it also covers the ottoman. The hand-stitched seam should be toward the back on the bottom. Now you have a chaise lounge that extends off the chair or couch.