Valve lash is pretty easy to do. In principle, you run the particular cam lobe into its base circle (by way of rotating the engine). In practical terms, that means that the push-rod of a lister should be at its LOWEST position. From there, you'll be measuring the gap between the NOSE of the lifter and the valve cap. You'll want a feeler gauge of ~ 0.008" to JUST slide between the nose and the cap. Should you not have that, then field expediency would demand two pieces of common notebook paper (~ 0.008"). Adjust the valve screw until that feeler gauge or paper is JUST touched (won't slide). At that point, tighten down the lock nut on the adjusting screw.
Now, move onto the OTHER valve. You'll have to rotate the engine to get the opposite valve into its base circle. Certainly, when the valve you JUST adjusted is ALL THE WAY UP, then the second valve must be in its base circle. Repeat the process with the feeler gauge or paper on the second valve.
Now, if all this remains too confusing, I'd recommend you put the Lister down for a while, and pick up a book about adjusting valves. I don't mean to be derogatory here. Just factual.