Use a Mag-Drill to drill the 3/4" holes each place where the horizontal members overlap the vertical members.

Before drilling, double check that when the holes go thru the vertical members, they will be on the lip of the U-channel, not going into the wall, or off of the lip.

Bolt the horizontal members tightly to the vertical members.

Flip the jig over so you have access to drill the other holes. See video for help.