Using a digital clamp meter is an uncomplicated and safe way to measure current in a circuit. Unlike regular meters, the clamp type does not put the meter into the circuit, but measures the field induced by a current passing through the clamp. It is suitable for moderate (<480V) voltage and current applications.
Remove power from the circuit, if possible. Working on a hot circuit involves extra risk to the user and all equipment involved.
Isolate a wire carrying the current you wish to measure.
Open the clamp portion of the meter and then close it around the wire (and only that wire) you wish to measure current passing through.
Set the clamp meter to read the current type (AC or DC) and the expected range in milliamps or amps. If you don't know the range, guess high, and dial down as needed.
Return power to the circuit and read the digital display, writing the value down to avoid repeating the process. If the reading fluctuates, give it a minute to settle down.
Remove power from the circuit. Then remove the clamp meter.
This type of meter is not for measuring current on a circuit board or for measuring small (<100 milliamps) current. This type of meter is not for precision current measurement. If a precise current reading is required, use an in-circuit meter.
When working with electricity, always use proper safety techniques.