Insert the new potentiometer in the same slot as the old one came out of.

Note that the potentiometer has a small clip that neatly falls inside the upper part of the slot. Make sure the potentiometer is oriented so that its clip falls into this slot as well.

Put both the washer and the nut screw back over the potentiometer, and tighten them with the wrench.

Grab the knob of the potentiometer with two fingers, and turn it counterclockwise until it stops.

Place the black cap back on top of the potentiometer, with the white indicator at the start of the green PLA section.

Once mounted, turn the black knob clockwise all the way, and verify that it is at the end of the dark-red PA section.