Remove the existing upholstery with a seam ripper, claw hammer and pliers. Take apart the cover at each seam and make notes on how the pieces are sewn and how they were attached to the frame. Draw simple sketches and take digital photos to help make the attachment process easy to follow.
Iron the seams on the existing cover so that each piece is flat. Trace out the pattern of each piece on paper and cut out your paper patterns. Label each pattern piece as you create it and note any darts, pleats or other more difficult sewing elements. Many ottomans have welting between the seams. Welting is a cord wrapped in fabric. If your ottoman has welting, you will need to make the welting first.
Make welting by measuring the length of cord you will need to wrap all the way around the ottoman. Place your decorator fabric on the cutting mat. Cut fabric strips on the 45 degree angle. Each strip should be 1-1/2 inches plus the diameter of your cord wide. Example: a bias cut is created by placing a square of your fabric flat on the table. If your fabric is 45 inches wide, then the square would be 45 inches long. Cut from the top right corner to the bottom left corner.
Attach your strips together. Place one strip vertical and face up. Place a second strip face down and horizontal with the leftmost side covering the bottom edge of the vertical strip. Sew from the top left edge to the bottom right edge and trim the seam close. Make a continuous strip long enough to cover all of your cording.
Place a welting, piping or zipper foot into the sewing machine. Fold the fabric wrong sides together along its length. Insert cord into the fold. Line up the raw edges and sew as tight to the cord as you can. Set your finished welting to the side until you need it.
Pin your pattern pieces to your decorator fabric and cut out each piece with scissors. Sew any darts or pleats first. Sew your pieces together in the order they need to be attached. Usually this is the top padded piece first. Sew the side panels into a circle. If the edges have welting, you will need to sew your welting between the seams. Align the side panel seams to the cover corners face sides together. Insert the welting with the cord toward the middle of the cover and the raw edges aligned with the raw edges of the cover and side. Start halfway down one side and don't sew the first 1/2 inch of welting.
Sew the seams until you are 3 inches from where the welting overlaps. Cut the welting 1 inch too long. Open the seam on the welting 1 inch. Cut just the cord even with the end of the cord already sewn. Fold the end of the fabric under 1/2 inch. Butt the ends of the cords. Slip the folded fabric over the start of the welting. Pin. Sew the remaining seam. This will hide the cord ends and look good. Use this welting technique for any welted seams.
Pull the new cover over the top pad of the ottoman. Often the new cover will be nailed by brads to the wood frame. Use brads and a hammer and nail the tacks close together or every 1/4 to 1/2 inch. Sew the bottom panels together in a large circle. It may look good if you add welting to your top edge. Slide the inside-out sides over the attached cover. Align the corner seams. Position the top edge just above the brads. Hammer a new row of tacks or staples close together and straight around all sides of the ottoman. If you added the welting, your edge will look very nice as you pull the cover over the ottoman frame, turning it right side out as you pull it into position.
Turn the ottoman upside down. Bring your fabric around to the underside of the ottoman and staple or tack the fabric into place. If the bottom is open, you should staple black fabric with the ends tucked under over the bottom to hide your fabric staples.