Slide the shaft in, small end first, until it just protrudes from bearing on the gear side.

Hold the pinion gear in place and slide the shaft the remainder of the way into it.

Make sure the shaft is flush with the back of the machine and the pinion gear is on as far as it will go.

Tighten the set screws on the gears (there are 2 on the pinion).

Nylon gears are supposedly self lubricating but I find a little sewing machine oil is useful since it is driven by a worm gear.