Sand the slots under the seat of the stool with the sandpaper to remove residue from the original glue.
Apply the wood glue inside of the holes. Make sure to coat the bottom and sides of the slots well.
Insert the ends of the stool legs into the slots and clamp them securely to the base using the wood clamps.
Stand the stool upright and place the level on the seat. Make any adjustments to the legs to level the stool before the wood glue dries. Allow the glue to dry for 24 hours prior to use.