Set the ottoman so a long side is facing you if the ottoman is rectangular. Place the measuring tape end on the floor in front of the ottoman and measure over top to the floor behind. Place the measuring tape on the floor at one of the short sides of the ottoman and measure over top to the floor on the opposite side. Add 1 inch to both measurements and write down the numbers. The side facing you for a square ottoman does not matter.
Cut out the fabric using the increased measurements and tape the fabric with the right side down to the ottoman with several loops of painters top attached to the top surface. Verify the fabric is hanging evenly on all sides of the ottoman before proceeding.
Hold a corner of the fabric at a diagonal from the ottoman so it is even with the top surface. Pin the front and side ottoman fabric together along the edge of the ottoman to fit the cover. Release the fabric and repeat with each corner to create a fitted cover. Verify the seams line up with the corner edges and the cover lays flat on the ottoman. Adjust the fabric and pins, if needed.
Remove the fabric from the ottoman and sew a seam along the pin lines at each corner. Pull the cover on the ottoman to verify the fit. Adjust the seams and fit of the cover, if needed.
Take the cover off the ottoman and cut off the excess fabric at each corner to leave a seam edge of about one-half inch. Sew the raw seam edge with a zig-zag or serger stitch to help limit fraying and increase durability.
Turn the bottom edge of the cover one-quarter inch to the inside and pin to hold. Turn the edge under another one-quarter inch and pin to hold the hem. Sew the hem with a straight-stitch to secure and finish the cover.