I find that it is easiest to put the gears on in the reverse order.

1. First take the gear set on the machine off and loosen the socket head Allen bolt  that holds the swinging arm in place.

2. Put the D gear on first and fasten it on the shaft. The D gear is the one on the  lead screw.

3. Then put the C gear on. It goes in the middle position. The stud it mounts on  slides back and forth. Slide it until it meshes with the D gear.

4. Then put the B gear on the same shaft as the C gear and put the c-clip back on.

5. Put the A gear on the top shaft and put the c-clip on.

6. Swing the gear assembly up so the A & B gears mesh. Tighten the lower socket  head Allen screw so they stay together.