This can be a tricky job. Are you certain that you have a solid lifter cam? If so, you also need the spec sheet on it. The valve lash can vary from cam to cam and without the specs, you can wipe out the lobes if adjusted incorrectly.
If you have the specs, you do adjust them while the engine is hot. Remove one valve cover at a time. You'll want to get a remote starter switch and hook it up with the car in neutral. Bump the engine a little at a time while viewing the pushrods. To make it simple, do one cylinder at a time. Once you see both pushrods (intake & exhaust) are in the closed position, you use a feeler gage to set the gap between the rocker arm and valve stem while loosening/tightening the rocker arm nut. Go back and double check each cylinder twice.
'69 Z11 Pace Car, 350/4 speed, manual top, console w/gauges, tilt rosewood wheel, AM radio