There are a couple ways to do it, but here is a simple noob proof way to do it. Basically you want to set the valves on the cylinder that is @ top dead center of its compression stroke. If a valve is partially open, you cannot set its clearance (duh). So how can you be sure you are @ TDC on the cylinder you are setting? One way is to start on #1 cylinder and line up the pointer on the crankshaft, but you dont know if its TDC on compression stroke or exhaust stroke. So you remove the spark plug from #1 cyl and place your finger on the hole while someone else either turns the motor with a ratchet, or gently bumps the starter. When you feel some pressure, you know the pointer mark is close and will be TDC compression stroke.
The idea here is to loosen the 5/8 nut a bunch so the rocker is totally loose. Then while rotating the pushrod between your fingers with one hand, tighten the nut slowly with your other hand until some resistance is felt. A general rule is that if the lifters/cam are used, turn the nut 1/2 of a turn further, if they are new parts then turn it 3/4 of a turn ( the used parts are already worn in and shouldnt need as much lash correction later). Do the same for the intake and exhaust rockers on that cylinder. Now how do we line up the next cylinder? Since its an 8 cylinder 4 stroke motor, we know that the next cylinder in firing order will be @ the proper position in exactly 90 crankshaft degrees from the prior one. All we do is turn the crank 1/4 turn and we are ready for the next one.
The firing order for a small or bigblock chev(or a smallblock mopar for that matter)is 1-8-4-3-6-5-7-2
Just set the valves in that order, with a quarter turn of the crank between each cylinder and it should all work out. When your done, crank it around a few times and give them all a bit of a wiggle. Its normal if some wobble a bit but if any are rattly loose you have done it wrong. Once the motor is up and running you can reset them if its a bit ticky. Its easy to do while the engine is running, just a bit messy. You just loosen one until it starts to clatter then you turn it in 1/2 -3/4 turn and move on to the next (in any order).
If anything there doesnt make sense, just lemme know so I can clarify.
