Turn the power off on the circuit you wish to test. The device you are testing should also be unplugged, in the event that there is a malfunction with the on/off switch.
Set the measurement to the appropriate setting on the amperage meter (ammeter). For example, if you are testing DC voltage (direct current), AC voltage (alternating current), or DC current turn the adjustment knob on the face of the tool so that the tool is measuring with the correct scale. On most meters the scale is easy to read, and is incorporated as part of the adjustment knob on the face of the tool.
Plug leads or probes into the meter. The probes are the long pencil-looking devices with a wire from the end of the probe leading to the to the connector which attaches to the meter. Plug each of the probes into the meter in the correct port. The ports are usually color-coded with the probes, and are located on the side of the meter.
Turn the meter on using the on/off switch usually located on the top or side of the meter. Test that it is working correctly and that the batteries are fresh by touching the two probes together. You should see the needle in the window of the meter jump as the probes are touched together.
Turn on the power to the circuit you wish to test, then turn the device on. Touch the red probe running from the meter to the positive wire ("hot" wire) lead in the circuit. Next, touch the black probe running from the meter to the negative wire ("ground" wire) lead in the circuit so that both probes are touching the circuit simultaneously.
Look into the face or window of the meter, and you will see the amperage displayed in either digital form or analog (needle) form depending on the meter being used. In either case, the meter will display the amount of amperage running through the live circuit. Disconnect the probes and remove them from the meter when you are finished. Wipe down the meter with a clean rag before storing it for future use.