The rear sights found on the USGI M1 Garand and USGI M-14 (also M1A and Chinese clone M14/S variants) are all identical and work the same.
The M1 Garand "peep" style rear sight is simple and very accurate. As you hold the rifle in firing position the ELEVATION KNOB that adjusts for the VERTICAL strike of the bullet is on the LEFT. The WINDAGE KNOB that ad- justs for the HORIZONTAL strike of the bullet in on the RIGHT. The comb- ination of these two controls will adjust the strike of the bullet on the target in any direction.
Look at base of the sight and you will see a small scale on the receiver and a corresponding witness line on the sight cover itself. Using the WINDAGE KNOB (on right) move the witness line until it aligns with the middle line on the scale. Each "click" of the knob will move the strike of the bullet on target 1" for each 100 yards distance to target in the direction that you move the sight....right or left.
Now turn the ELEVATION KNOB toward the front of the rifle and lower the sight down as far as it will go. Again, each "click" of the knob will move the strike of the bullet on target 1" for each 100 yards distance to target in the direction that you move the sight....up or down.
You are now ready to zero your rifle.
You can do this mechanically if you have a chamber inspection mirror and a rifle vise handy. Clamp the rifle in the vise and using the inspection mirror BORE SIGHT the rifle on a target point at some distance away, 100 yards would be ideal. Now, WITHOUT MOVING THE RIFLE, adjust rear sight so that it also is zeroed on this same target point. Use ELEVATION KNOB (left) to raise sight and WINDAGE KNOB (right) to move right or left as needed. When done your rifle has a mechanical zero....you are ready to go to rifle range and get your true zero by actually firing for effect.
When firing to zero you must have the rifle in a good, supported firing position. This means shooting from bench rest with rifle supported or from prone position with rifle supported (sand bags, etc.) You will not be able to achieve a decent zero from any other firing position so don't even try....it would be a waste of time and effort.
Set up target at 100 yard distance. If you've set a mechanical zero then fire three rounds and then check target to see where they've hit. If you have not set mechanical zero make sure the WINDAGE witness line is aligned with middle line on WINDAGE scale and ELEVATION is fully down. Now fire three rounds and then check target to see where they've hit. If the rounds hit in dirt below the target then crank in approx. 6 CLICKS of ELEVATION and fire three more rounds. Check and see where they strike target.
BTW, a good spotting scope during this period will save some shoe leather.
Continue firing three rounds, check the target and then make adjustments on your sights until the rounds are striking exactly where you want them on the target....i.e.- the 10X bullseye.
Just remember....the LEFT KNOB adjusts for ELEVATION in 1 minute of degree increments and the RIGHT KNOB adjusts for WINDAGE (horizontal movement) in 1 minute of degree increments.