Remove the valve covers and spark plugs. Start with cylinder #1, closest to the flywheel. Rotate the flywheel by hand until the piston is at top dead center on the compression stroke, both valves will be closed and you can watch the piston through the spark plug hole. You can also use a piece of stiff wire like a welding rod or coat hanger, drop it into the spark plug hole and watch until it stops rising.. Rotate the flywheel another 1/4 turn.
Loosen the lock nut on each rocker and move the adjuster until a .005" feeler gauge leaf will move between the rocker and valve with a slight drag. Without moving the adjuster, tighten the lock nut and re check. You may need to repeat the procedure a few times until the feeler gauge drags slightly with the lock nut tightened. Install the valve cover with a new gasket and you are good to go. Repeat on #2.