Do it with the engine warm, remove the plugs, place back in the spark plug wire caps and ground them, wire tie the throttle wide open, make sure the kill switch is off, pull on the rope hard and as many times as it takes until the gage will not increase in pounds any more - look up the spec for that engine what the difference between the two or three cylinders should be.
You could do it cold but you will not get as good reading as warm.