Black battery version, or the older grey mains powered model? The method is essentially the same. Determine a good print exposure for an average negative using test strips. Using the same lens aperture, hold the diffuser under the lens and rotate the dial until the neon just strikes (old model) or the high/low LEDs are even (new model). Holding the dial still, adjust the exposure time ring so that the time matches the exposure time determined by your test strips. The alphabetic letter gives you the paper speed for future reference with this calibration.
You can use the thing as a spot meter for shadows without the diffuser, but you have to be careful about the slot masking the sensor.
I may have a manual (well instruction sheet) around somewhere.
