Reminder guys, that the typical Ammeter is a SHUNT ammeter, which means it will drop a small voltage when measuring current, stuff won;t add up properly unless you measure the voltage across the shunt and the added resistor. The Voltage burden is USUALLY in the specs.
A more complex method is to use a feedback ammeter. Pretty easy to design for currents below 10 mA or so. When your above 1 mA then lead resistance starts to get in the way of an accurate measurement.
Measuring the voltage across and the current through the resistor is the way to go for low value resistors. When you place a 10 m meter across a 10 Meg resistor, then it won't measure properly. It's a lesson in itself.
You can also indirectly measure the input Z of meters with a series potentiometer, a voltage source and an ohmmeter.
The Harbor Freight voltmeters are really cheap. They just don't turn off automatically.