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Bring the side tracks over the top and push down to put in place. Make sure the side tracks are level, going up and down, or the doors won't work properly; place a level against the track and center the bubble for a correct fit.
With the jamb flat against the wall, confirm that the track is level and temporarily secure it in place with painter's tape; do the same for the opposite side wall
When the side jambs are in position, use an extra small masonry bit (about 1/8" will do) to drill pilot holes for the screws that will fasten the jambs to the side walls. After drilling the holes, tear off the tape and remove the two side jambs and the bottom track.
With the framework removed, widen the pilot holes to 3/16". Using a 3/16" masonry bit, widen all the pilot holes and insert the plastic screw anchors. Attach the jamb closest to the storage column side with three screws. For the opposite side jamb, fasten just the top and bottom holes. For the middle hole, put a screw through a bumper insert, push it into the hole and tighten.
The side tracks are up, so snap the bottom track into place, making sure it's lined up along the edge of the tub, on the mark made earlier. This is where the extension piece comes in handy; extend it to the outside track to know when you are in position to mount the shelving tower. Remember, you want to make sure this gap is below the shelving tower so it's hidden. Tape is helpful here: it lets you double-check that you're on the right side.