Remove the mower deck. Block the blade from turning by using a piece of wood wedged between the blade and the mower deck. Loosen the nut on the top of the pulley (May require a cheater bar). Remove the pulley. If it doesn't come off easily, use a 3 armed pully remover. Once the pulley is removed, loosen the four self-tapping bolts that hold the spindle to the bottom of the deck and the spindle will fall loose. Remove the blade. Install the new spindle from below using NEW self-tapping bolts to bolt the new spindle to the deck. Replace the blade, install pulley, block blade from turning then tighten the top nut.