Place a large piece of paper on the floor or set down a sheet of newspaper. Flip the stool so the seat is resting on the paper. Trace around the stool seat to make a pattern.
Add 3 inches around the traced pattern to make a pattern for the fabric cover. Cut out a piece of medium-weight fabric using the increased size pattern.
Cut out the traced pattern of the stool seat and place it on top of a piece of 1-inch-thick foam. Trace the pattern onto the foam. Cut out the foam with a sharp knife.
Lay down the fabric cover so the wrong side is facing up. Set the piece of foam in the center of the fabric and put the seat of the stool on top of the foam. Inspect the foam so it's the same size as the seat. Cut off excess foam that hangs over the edge.
Grab a section of excess fabric and pull it snug to the under side of the seat. Affix a staple through the edge to hold it in place. Repeat this process around the entire seat. Flip the chair over so the foam cushion and fabric remain smooth while pulling the fabric taut.
Apply a stain protector fabric spray to the fabric on the stool after completing the project.