Motor Support Plate
When you look at the motor support plate, you'll see that it's got a "right" side that goes outward and a "wrong" side that lays against the front board. The "right" side has reliefs that look like countersinking for the screws that support the motor.
The bearing block to be mounted on the front board is either of two similar parts. They're black machined urethane castings. I didn't see any significant difference between the two of mine. The important thing to note is that the large opening in the middle is stepped, it's bored out to two different diameters. Make sure the size with the small bore goes against the board, the large bore goes away from the board. The same will apply on the back board's bearing block.
Attach the Motor Mount Plate and Bearing Block
These two parts are held on with a pair of screws that go through the motor mount plate, front board, and bearing block in that order. A pair of washers go on, then nuts are then secured to the screws, but only finger-tight for now.
Shaft End Mounts
There are 8 pieces that support the ends of the shafts for the X and Y axes, shaped like little cups. A machine screw slips into the open end and through the bottom. Then put the screw through the holes on each side of where the bearing block is mounted on the front board (see image, above. Click on it to enlarge it.) A washer and nut goes on the other side. Again, they should be secured no more than finger-tight, and not at all tight at that.
Delrin Nuts Pre-fit
The Delrin nuts are a pair of round plastic pieces that fit into the round cutouts at either end of the front board. This is a good time to pre-fit them, so that attaching the front board to the side boards will be easier later.
First, test-fit the screws into the nuts. In my case, one screw fit one nut better than it fit the other, the other screw didn't care which nut was on it. Next, put the nut part way into its hole in the front board with the screw still in it. Line up the shaft of the screw with the hole in the side of the board. That will help line up the nut for the screw before you push it into place. In my case, I also put a small alignment mark on the nut and the board. Then take out the screw and push in the nut all the way, carefully keeping it aligned as well as possible.
Once it's in, run the screw back in through the hole in the end of the front board to keep it in place until we attach the front board to the side boards. Do this on both sides, of course.