Why ohms cannot find a bad ground
An ohmmeter measures resistance with a tiny test current — microamps. A ground strap that reads 0.2 ohms cold and unloaded can develop 3 ohms of effective resistance when you push 40 amps through it because heat, corrosion, and loose fasteners behave differently under load. Voltage drop tests the circuit under real operating current, which is the only measurement that matters.
The method in three steps
Set your meter to DC volts. Put the red lead on the power source (battery positive for a power side test, battery negative for a ground side test) and the black lead on the load side of the circuit component you are testing. Turn the circuit on so current is flowing. Read the meter. Anything under the SAE spec for that connection is a pass. Anything over is your fault.
Where technicians misuse it
The test only works with current flowing. Testing a starter cable with the key off tells you nothing. Crank the engine and read the drop during the crank event. On modern cars with computer-controlled circuits, use the scan tool to command the load on if you cannot key it on directly.