I'll try for a real basic, simple explanation... Shoot at the top of your target (preferably in the middle as well) from 20 yards using your 20-yard pin. Tie a string with some weighted material at the end to keep your string hanging straight down (something like a threaded nut would probably work perfectly...).

Walk back at least 10 yards, and -- using your 20-yard pin again, aim at the first arrow. Walk back at least 10 yards again, and -- again, using your 20-yard pin, aim at the first arrow. You can keep walking back and repeating this until you run out of shooting room.

The purpose of the string is to help you see if your arrows are falling perfectly in line, and if you have your bow tuned perfectly for centershot. The second, third, fourth (and any other arrows you've shot) should be in a direct line below the first arrow.

Hang a weighted string from a nail on a target. Stick a round sticker on the target face so that the string splits the sticker. Use your 20-yard pin, step back 20 yards from the target andshoot at the sticker. Don't worry about where the arrow hits.

Walk straight back to 30 yards, and using the 20-yd pin,shoot an arrow at the sticker. Repeat at 35 yards and at 40 yards, using the 20-yard pin andshoot at the sticker.

If your arrows look like this pattern " / ", then move your arrow rest 1/16th inch to the right. If your arrows look like this pattern " \ ", then move your arrow rest 1/16th inch to the left.

Repeat all of the above steps starting from 20 yds, etc.

Eventually, your arrows will hit in the target in a straight up and down line like this: " | ".

But, your vertical pattern of arrows may not be hitting the string. If not, and the arrows are say six inches to the left of the weighted string, adjust your windage on your sight ring until you can nail the string at all distances.

Lock down the windage and lock down the arrow rest. Windage and center shot are now perfect.