Using the 2mm Allen key, install the M4 grub screws into the threaded openings on the sides of the coupler.
Just start the thread so that the grub screw holds in.
Verify it is seated properly and won't fall out.
The top part should be flush and there should be bottom part of the stopper visible from the side.
Before you start installing the motor, the flat part of the motor shaft must be aligned with one of the two grub screws in the metal coupler.
Orient the coupler so that one of the grub screws is pointing up.
Insert two M3x10 screws into the marked openings.
Start the thread so that the screws engage into the motor. Don't tighten them up all the way yet!
From the opposite side, add another two M3x10 screws into the openings. Push them towards the motor. Using the ball-end 2.5mm Allen key at an angle, tighten them up fully.