Power on the electric circuit.
Select a component you want to measure the voltage of. For example, you may want to know the voltage across a particular resistor. A resistor has two pins on either end, which represent the ends of a water pipe. The voltage across these pins represents the pressure differential according to the electronic-hydraulic analogy.
Attach one probe of the voltmeter to one end of an electrical component on a circuit. A voltmeter has two probes: one red and one black. Typically, the black probe is attached to the low voltage side of a component and the red probe is attached to the high voltage side. If you don’t know which end is which, use the red probe on an arbitrary end of the component.
Attach the other probe to the opposite side of the component. The voltmeter will display the voltage across this component. If the value is negative, you have attached the red probe to the low voltage end of the component. Since you are just measuring the magnitude of the pressure differential, the sign is unimportant.