As an experienced DIY upholsterer, I recommend flat-weave rugs with nubby textures and neutral colors to effectively mask spills or wear. Select a rug about four times wider than your ottoman for sufficient material and waste.
Use a drill or screwdriver to detach the ottoman cushion from the wooden legs or frame. Then, carefully cut away the existing fabric with dressmaker's scissors.
Measure the ottoman cushion's length, width, and height with a tape measure. Mark the rug with a marker or chalk, then cut to size using dressmaker's scissors. For box cushion ottomans, also measure side heights to cut matching panels.
For a snug, smooth finish, add one or two extra layers of batting. Measure, mark, and cut to size, then secure to the cushion backing with a staple gun and staples.
With a helper, stretch the rug taut over the batting and cushion back, securing it with a staple gun and staples. For box cushion ottomans, sew side panels to the top using a ballpoint tapestry needle and a 2-ounce skein of matching mercerized cotton. Finally, reattach the cushion to the frame or legs with a screwdriver or drill.