Voltage drop testing an entire circuit usually begins at the positive side of the circuit, but it can begin any where it is most easy to get at, start with the easy areas first.
(positive probe red arrow, negative probe black arrow)
Voltage drop testing must include every connection possible, or you may miss what you are looking for, make sure you don't miss any, know exactly where the resistance or open exists.
Voltage drop testing wire will reveal partially broken, or corroded wiring internally.
Voltage drop testing a switch will see any possible point resistance.
Voltage drop across the component should equal battery voltage.
Checking available volts is a fast way to check for drop across the positive side circuit, this should be close to battery volts.
Voltage drop testing ground side of circuit is the same except probes are reversed on this side. Just make sure positive probe stays towards battery positive throughout the entire circuit.
Test all connections on the ground side too.
Until you,
End up at the end of the ground side circuit.