How do you adjust valves in a 1968 Chevy 250 6-cyl?

Same as the Chevy V8's. Warm engine up, remove valve cover, PCV valve plugged so the engine will idle, let engine idle, back off first rocker arm nut until it clatters, turn back in just until the clatter stops, continue to turn the nut slowly 1/4 turn in at a time for one full turn in. First one done. Now do each one the same way. This job can get a little messy with oil splashing around so work quickly as you can. There are clips available that snap onto to the rocker arm to deflect oil from squirting out of control but you still have a messy job even with those clips.