Put the shaft assembly into the backpack, making sure to press it fully into the bottom hole.

Fit the upper mount so the bosses fit into the holes in the backpack, and screw the inner mounting plastic part into place. Screw in the three upward facing screws first, that way they pull the plastic part up and get the back facing screw holes lined up better.

Install lower screw, zip up, press hook and loop together, and you're done.