Set your idle speed as low as you can and still keep the engine running. Holding a socket wrench loosely in one hand, turn the rocker arm adjusting nut while "feeling" the clearance with the appropriate feeler gauge (the angled ones are easier to use than the flat ones). If you use a breaker bar instead of a ratchet, you can turn the nut either way without having to remove the wrench to reverse the turning direction. You might want to stuff some rags around the head to catch any oil that might splatter. Don't worry, it's not going to be spraying wildly everywhere.
While the rocker arm is going up and down like a sewing machine, place the feeler gauge between the arm and the valve stem and attempt to slide the gauge in and out. If the lash is too loose, the gauge will slide easily with no resistance. But if the lash is too tight, you will feel some "drag" on the gauge. With the wrench bouncing loosely in your palm (hand held above the rocker shaft), make the appropriate adjustments on the nuts to leave just a SLIGHT bit of drag. After doing a few rocker arms, you will get a feel for the proper amount of drag for the given gap you are looking for.
Good luck,
Jerry
