Step 1
Turn the ottoman over so the bottom is face-up. Unscrew the leg on which you want to add a caster. You can also install the caster right onto the leg without having to remove it.
Step 2
Place the casters against the bottom of the legs if you have not removed them. If you have removed the legs, place the casters in each of the four corners of the ottoman. Mark the four screw holes with a marker so you can drill pilot holes.
Step 3
Use a drill to drive pilot holes through the marks you made. If the ottoman has a hollow middle, do not drill holes through the mark over that part. You will not put a screw there.
Step 4
Position the casters so that the holes line up with the pilot holes. Place a washer over each hole then drive the screws in place. Turn the ottoman upright.