With the engine warmed up but turned off, remove the valve covers. Start the engine and loosen the rocker arm nuts until they make a knocking sound. Slowly tighten the rocker arm nuts until they stops knocking. Tighten it 1/4 to 1/2 of a turn past this point and they should be set. DO NOT OVER TIGHTEN THE NUT OR YOU WILL BEND A VALVE! Turn off the engine and replace valve covers. Check the oil once the engine is cool.
You may modify an old pair of valve covers by cutting open the top, so you have a window through which to adjust the valves. It will still be messy, but it will reduce the amount of spilled oil.
What? You don't adjust your valves with the engine running! You move the crank with a wrench so the cylinder you're adjusting the valves on is at TDC (or the valve push rod is in the full down position), insert a .015 feeler gauge between the top of the valve and rocker arm. Tighten (rocker nut) until the clearance of the feeler is met but not so tight that you can't remove the feeler tool.