Take 1 M3 x 35mm and put 1 washer on it. Insert it into one of the two holes opposite the motor mount.
Slide the screw through the hole in the platform spacer. make sure the flat part is down.
Place another motor end on the screw after the platform spacer, followed by 1 washer and 1 ny-lock nut. Do not tighten the nut all the way, keep the nut loose, in order to attach the other two platform spacers. Repeat steps 1-3 for the rest of the motor mounts and spacers. Once complete, tighten all the screws and nuts.