I will make it short and easy to understand. #1 Start your engine so it go to normal operating temperature #2 Stop it and let it cool down for 10 minutes before removing spark plugs and very important you must remove them all before the test. Why? Suppose that you have a cracked head gasket between two cylinders when you will do the test you may not remark the lost of pressure because the cylinder to the next you are testing still have his spark plug to keep the pressure in the engine. #3 Disconnect the electrical wire that plug onto the coil pack. Never only unplug the spark plug wires because great injury or damage could happen if the 40,000volts spark hit you or a component of the truck #4 Do the test on each cylinder and be sure the hose is tight in the spark plug thread because a lost of compression could happen and causing a misreading of the results #5 Crank the engine so it could rotate 7 times and read the results.If you have time do it twice on each cylinder just to be sure you correctly did it. #6 The needle on the gauge should rise up quickly on the first rotation of the engine and continue to rise steadily. If at first rotation the needle stay almost the same or only goes up a little and after that it rise up normally then you have a ring problem.To test the rings pour a little amount of oil in the spark plug hole then replug the tester and make the test.If the first crank rise the gauge quickly then you are sure the rings are bad. Also at first rotation and each other after if the needle dont rise quickly then you have a valve problem.Maybe burned valve,deposits that make it not close correctly or craked head or even maybe blown head gasket.After the test of oil if no compression is present then you know it's not rings but valves,gasket pr cracked head. Ford specs tell 75psi maximum of difference between the highest and lowest cylinder reading and a minimum of 100psi in each cylinder. In exemple I just tested mine today because the engine had 130 500 miles. Cylinders 1=168 2=180 3=160 4=167 5=172 6=167 As you can see all my cylinders are far more than 100 psi and have only 20psi of difference between the highest and lowest reading. TEST PASSED!