Run your incoming signal through two 16-ohm resistors. Connect a wire to this initial wire that goes to a third 16-ohm resistor. Further down the initial wire, connect a wire that goes to a 1N60 Germanium diode. After that, on the initial wire, connect a wire that goes to a 5600-ohm resistor and then to a switch point on the SPDT switch. This is your calibration circuit.
Connect another wire to the initial wire from Step 1 that leads to a 4-microfarad capacitor, and then to a switch point on the SPDT switch, adjacent to the switch point in Step 1. This is the modulation circuit.
Connect your RF choke to the SPDT switch, opposite the two switch points from Steps 1 and 2. Run a wire out the other end of the choke and run it to a 50K carbon pot resistor. Before the resistor, add a wire that goes to a 0.005 microfarad capacitor. Continuing on from the resistor, add a wire that goes off to a 1N60 Germanium diode. Connect another diode of the same type and size to the end of the wire that connects back to the choke. After this diode, start the wire again. Connect it to another wire that serves as a "T," headed toward a .005 microfarad capacitor on one side and your modulation meter gauge on the other.
Set the SPDT switch to the calibration point, closest to the 5600-ohm resistor. Turn on the carrier current and read the meter, but do not talk into the microphone. Turn the calibration control on the meter until it tells you that the current is at "Full Scale."
Move the SPDT switch to the modulation point. Turn on the microphone and speak into it. The needle on the gauge will move with each word you speak; the amount it moves will tell you the percentage of modulation.