Bring the motor up to TDC on # 1 cylinder on the firing stroke and test it, rotate the crankshaft 90 degrees clockwise to test #8,and another 90 degrees to test #4 and test, then rotate another ninety degrees to test # 3 and go through the firing order in sequence to test the rest, one at a time (Imagine straight up as 0 degrees and 1/4 turn is 90 degrees
Most car motors in the car are hard to get 90 degrees rotation with one movement, if you stop more than 5 degrees before or after TDC the air pressure will probably blow the piston down to bottom dead center, make sure and remove the ratchet or breaker bar from the socket or crankshaft bolt before each test If you don't you may regret what it breaks or damages, like your arm or radiator shroud
The leak down meter will come with instruction, some are better than others, I own two, one old single gauge from Moroso that quit reading accurately years ago(it would stick ) and a newer one from Aircraft Spruce for testing my old airplane motor, it is a dual gauge tester and easier to use