Electrons in a circuit are pushed around by what's called the electromotive force (EMF). The more common name for EMF is voltage, which gives energy to the electrons. The amount of energy doesn't depend upon which direction the electrons are being pushed, and that's why an electrical system can have either a positive or negative ground. Electrons moving in either direction can, for example, light headlights or swipe windshield wipers. Test equipment also doesn't much care which direction is toward "ground."
Switch the meter to its resistance setting and measure the resistance of test points in the circuit. Do this step before you power on the circuit. Not every circuit will have well-defined resistance test points, but you can often detect significant problems, such as a short or open circuit, by measuring resistance. There is no difference in the method of measuring resistance between a positive and negative ground system.
Set the multimeter to the voltage scale. You may find it helpful to place the positive lead of the multimeter on the ground side of any connection, but it's not critical. Just remember that a multimeter measures the voltage difference between the two selected points, and that the sign of the voltage difference will be different in a positive and negative ground system.
Configure the circuit to measure current at your selected test points. Current measurements require you to replace a section of the circuit with a current path through the multimeter. For example, you could unscrew a wire from a terminal lug and place one lead of the multimeter on the terminal lug and attach the other lead to the wire. This way, any current needs to go through the multimeter to reach the circuit.
Configure the meter to measure current. There are separate input jacks on the face of the meter that are designed to take input current. The leads must be plugged into the current leads. If there is more than one set of input jacks, use the jack that corresponds to the highest current you expect. You can always select the lower jack if the current is not as high as you anticipated.
Observe the polarity of the current input leads. Some multimeters require the polarity of the input current to match the polarity of the jacks. This is where you'll find a difference between positive and negative ground systems. The positive current input should be toward ground, while the common input is toward the circuit. Not every meter requires the polarity to be observed, but it's a good practice.
