Measure both the top and sides of the ottoman with a measuring tape. If you do not have a measuring tape, a yard stick will do just fine. A yard stick will also create a straight edge for cutting the fabric if you have difficulty keeping the cut line straight. Add an additional 1 inch on all four sides for a seam allowance. Lay the fabric out face down on the floor or other large flat surface. A dining room table works great for cutting fabric as well. Use the sewing scissors to cut the fabric according to the measurements.
Pin the top and side pieces of fabric together with sewing pins. Use a sewing machine to sew a 5/8-inch seam all along the pinned edges. Remove the sewing pins and turn the fabric right-side out. Fit the slipcover over top of the ottoman, pulling it down on all sides. Adjust the corners so that they are in alignment before turning the ottoman over to attach the fabric to the bottom. Flip the ottoman over on its top so that the bottom is visible.
Pull the fabric down over the edge of the ottoman on all four sides and place a staple in the center of each bottom edge of the wood frame to hold the fabric in place. Now start pulling the fabric down keeping it taut so that it doesn't wrinkle. Staple it in place as you go completely around the bottom of the ottoman. Place the staples closely together so that the fabric does not buckle. This will keep it in place and will help the fabric to resist coming off when in use.