Don,
Bruno is correct, the green wire on the throttle control must be cut, so that one end can be soldered to the centre pin, and the other end to either of the outer pins.
The voltage can be adjusted by turning the screw, but you will have to find out whether the gauge voltage increases or decreases with clockwise rotation of the screw, because this will be dependant upon which outer pin you have used.
(I think pin 3 might be better, so that turning the adjuster clockwise should increase the voltage)For a starting point with a 48V battery, adjust the trimmer until you have approx 75% of the battery reading for a 36V gauge, or 50% for a 24V gauge. If you're using a 36V battery, adjust the trimmer until you have approx 66% of the battery reading for a 24V gauge.
Unscrewing the adjuster should make the gauge lights go out sooner, assuming pin 3 is the correct one.
A bit of trial and error may be required to get it to work just right.
I hope that's made it a bit clearer.
Alan