Rotate the upper gear ring (the one you just installed) as far clockwise as you can

Rotate the lower gear ring as far counter-clockwise as you can.

Both of these gear rings must remain in this position while installing the reverse gears or the zoom ring will be misaligned with the zoom mechanics.

Insert one of the reverse gears into position, using caution to not move the gear rings.

The reverse gears are the 3 plastic gears that go in between the two gear rings.

Screw the two M1.7x3mm JIS #000 screws into the reverse gear.

Repeat the previous two instructions to install the other 2 reverse gears.

You may now move the gear rings to ensure they move smoothly.