Turn on the multimeter and set it to read ohms. You may need to adjust the multimeter's ohms range, if you are using an analog meter. The range is the defined scale of ohms; for example, you could set the range from 0 to 5 ohms. If you're not sure of the possible range, you can use trial and error until you get a good reading.
Connect the digital multimeter's probes to the negative and positive source wires in the circuit. These connections need to be made to bare metal. Record the number of ohms displayed on the meter. You can measure the ohms and amps in any electrical circuit, whether it be home, automobile or appliance, as well as a circuit board, if you understand the connections.
Set the multimeter to read current or amps. You may also need to adjust the range in the same manner as reading the ohms. Record the number of amps displayed on the meter.
Multiply the amps by the ohms. The result is the voltage available in the circuit.