The Tetrix clamshell bracket is sized to grasp the circumference of the gearbox. The mounting screw does double duty for holding the clamp to a beam or plate and tightening around the gearbox. The screw on the other end only holds the bracket to the beam. You need to put both screws in place first. Insert the motor gearbox first since it's got a rounded end that goes in easier than the other way. Then put your drive gear on the output shaft. You can now grasp the barrel of the motor and twist it so that the motor's gear meshes with whatever gear you're driving. Once you get a good alignment, you can tighten the clamp end of the bracket. I'm using the nut on the outside or top of the clamp with the cap end of the screw on the inside of the beam or plate. That way a wrench or nut driver has no trouble getting a grip on the nut. If you cannot get it tight enough, there is a chance you have stripped the threads on either the nut or the screw. These are only #6 hardware and practically all FRC team members are strong enough to over-tighten them.
HTH - good luck