Zeroing open sights on an AR is no different than zeroing the sights on any other rifle. The most important thing is establishing a zero that compliments the shooting you intend to perform.
Rear sights for ARs have a click adjustment feature. The value of each click varies depending on sight radius—the distance between the rear and front sight. Each click will equal about 0.5 to 0.75 inches at 100 yards.
Most rear AR sights have two apertures to choose from. Generally, the large aperture is used inside 200 yards and the small one for 300 yards and beyond. An offset between the positions of the holes makes this possible.
The XS Sight Systems’ CSAT replacement aperture for AR rear sights has a notch cut above the small aperture to be used as a rear sight for close-quarters-combat shooting.
Pop-up AR sights like these MBUS sights from Magpul are zeroed in the same manner as any other AR sight.
The A2-style rear sight, common on many ARs, allows for windage and elevation adjustment. Most have range indicator marks on the dial for quick reference.
Because the sights on an AR are positioned high above the bore (including backup iron sights like the Magpul MBUS), inside of 10 yards bullets will impact about 2 inches below the point-of-aim.
If you are zeroing an AR with an optic, open sights or both, the process is the same: Establish a close-range zero and then confirm and fine tune at longer ranges.