An insulation resistance tester (commonly called a Megger after a prominent company that manufactures them) is operated by hooking it up to a piece of electrical equipment and running voltage across the insulation on the equipment. The Megger calculates how well the electrical insulation on the equipment is working by checking how much current is flowing between the Megger to the machine. The higher the resistance, the less current will flow, amounting to better electrical insulation shielding. This is important, because poor insulation leads to equipment breaking down.
To test for insulation resistance, make sure to isolate the equipment from any other sources of current. Turn off the equipment, and disconnect it from other components. If it is not possible to completely isolate the equipment, observe what other conducting parts it is connected to, such as cables or wires, and be aware that the insulation resistance rating will be lower than due to these other components. Ground and discharge the equipment to prevent sparks or electrocution both before and after testing.
Attach two leads from the Megger across the equipment's insulation, with one lead on one side and one on the opposite side, and run the voltage on the Megger for at least one minute. The device will calculate the resistance using Ohm's Law and give the resistance rating in megaOhms.