It has been a while but I believe I did it with the valve cover off. There is a hex shaped section on the camshaft you can use to turn the engine. Put it a close to TDC on the cylinder you want to check and hit it with the air. If it doesn't move you did it, if it rotates either direction you need to turn it a little less, or more, until the piston stays on TDC with pressure in the cylinder.