Use a pick to adjust the variable resistor through the opening in the top plate.

Turn it until the needle in the viewfinder is centered in the opening, indicating a proper exposure.

The adjustment resistor shown with the top cover removed.

Repeat steps 3 and 4 for several different light levels.

Since the Spotmatic only has one adjustment resistor, it may not be possible to center the needle at every light level. Make compromises as necessary.