Turn the stool upside down so you have access to the legs. About 2 inches from the bottom of one of the legs, tie off a section of the sisal to the leg using a slip knot. The wood that remains uncovered will form what looks like a foot for the stool leg. The 2 inches of space also will keep the sisal from rubbing against the floor or carpet. Use a dab of wood glue to hold the end of the sisal against the chair on the inside of the leg.
Wrap the sisal around the leg of the stool toward its seat, covering the beginning strand of sisal and adding a drip of wood glue to the wood surface every few strands around to hold the sisal secure. Keep the strands tight together until you reach the top of the leg. Tie the sisal to itself with a slip knot on the inside of the leg, cut the excess with scissors and tuck the loose end under the previously wrapped sisal. Put a little wood glue under the knot.
Repeat the wrapping process for each leg, keeping the starting point at exactly the same height for each leg. Wrap the inside braces going from leg to leg if you want to cover all the wood.
Leave the top of the stool without sisal since the roughness of the material is hard to sit on and will only get rougher with time as extended use frays it.
Turn the stool over and wait for the glue to dry for about an hour before using.