Measure the length and width of the top of your ottoman as well as the height. Add one inch to each of these measurements for seam allowances. For instance, if your ottoman measures 13 inches square on the top and 18 inches in height, your final measurements are 14 inches and 19 inches.
Draw one square of fabric for the top of your ottoman and four rectangles of fabric for the sides that match your measurements. Cut out the pieces with sharp scissors.
Stitch the four side rectangles together along the edges with the wrong sides facing out. The pieces should form a hollow, rectangular tube.
Stitch the top square to the top edges of the rectangular tube. Turn the ottoman cover inside-out to reveal the right side of the fabric.
Gently pull the slipcover down over your ottoman. Smooth out the sides and poke the corners of the top of the ottoman into the corners of the slipcover.