The motor mount, if viewed from the rear of the motor, is designed to fit underneath the upper surfaces of the "T", formed by a long angle under the motor shaft (extending away from the viewer), and 1" angle attached to it.
The motor mount should be attached before the motor. Note that the central screw in the motor mount needs to be head up.
Prior to installing the motor, fit the spider coupling, making sure there is enough room for the motor shaft. Adjust the leadscrew/pulley shaft axially if needed, then install the coupling.
The motor is installed as follows: a screw is inserted from the motor side, a 1/4" washer (1/16" thick) is placed on the screw in between the motor and motor mount, and the nut is held on the motor mount while the screw is tightened with a screwdriver.
The motor alignment does not have to be perfect. Once the motor is installed and aligned, spider coupling can be tightened.