Connect one of the test light probes to the negative terminal of the battery or (ground) of the vehicle. Connect the other probe to a connection in the circuit being tested, preferably located as close as possible to the battery or fuse.
If the test light comes on, voltage is present, which means the part of the circuit between the connection and the battery is OK.
Continue checking the rest of the chain in the same manner.
When you find a point where there is no voltage, it means that the source of the problem is between that point and the last point where voltage was present. Most problems are caused by a bad connection.
Attention! Remember that in some circuits voltage is present only if the key in the ignition switch is turned to a certain position.