If things aren't stock, you need to be careful and do it correctly. First you need locknuts. You can google and find a procedure for rotating the crank a certain way and adjusting certain rockers. I do it the hard but simple way.

Find TDC for cyl 1. Mark the balancer at 3 additional 90 deg spots. Since both valves are closed when the particular piston fires, you can adjust the intake and exhaust for that piston. First adjust cyl#1 valves. Then rotate the crank to the next 90 deg spot and adjust the #8 cylinder, 90 more degrees and adjust #4, etc. Firing order is 18436572.

To adjust the lifter, back off the nut until the pushrod is loose and can wiggle up and down. Wiggle the rod up and down as you tighten the nut until you have just removed the slop. Then give it 1/8 to 1/4 more turn and lock it. I use polylocks with the setscrew. That 1/8 to 1/4 rotation is enough to lock the set screw as you turn the nut and setscrew at the same time.