I've found mounting the stepper gear to the stepper shaft can be the most difficult part.

Start by sliding the stepper gear on the stepper shaft so the set screw is on the flat side of shaft.

Mount the extruder mount to the frame with 2x M5x10 button head screws and 2x M5 T-nuts. Placement isn't an issue because this is only for putting the gear on the stepper shaft

Disassemble the BMG extruder and place the back half on the extruder mount. Make sure the orientation is correct. Place the E stepper motor in the mount.

Using 2x M3x20 socket head screws and 1x M3x30 socket head screw mount the back half and the stepper to the mount.