Ok, first start your motor and let it get up to operating temeprature. This will get you up to the compression you have when the piston is expanded and everythings hot. Then pull one plug and screw the tester in. Turn killswitch off, give it full throttle, and kick untill it levels of. Then take it out and put the plug back in, then repeat for the other side.
I prefer to leave the other plug in so nothing gets sucked in or falls in. Its only a little harder to kick.