"Rockover" method. pull plugs/turn crank w 1&1/4" socket & breaker bar. turn it until the both valves on #1 cyl are moving and are at the same height, at that point adj the clearance on #6. Then turn it until both valves on #6 are moving and are at the same height, at that point adj the clearance on #1. Then go to the next vertical column (8 & 5) & repeat(disregard the A & B) & then the next 2 columns. If the spec is for example .020", a .019" flat feeler gauge should have no drag and a .021" gauge should have light drag & with the gauge held square this will get you within .001" of your spec. EDIT forgot the diagram (it's Monday)