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.

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