You can download, from this forum or a Google search, a military 25-yd. sighting target.
(example: http://ohmr.ohio.gov/forms/ohmr2444-b.pdf). The black center will all but disappear at 25 yds., so aim carefully, center-hold.
These directions will work for a 16" M4 carbine or longer barrel (my AR has a 16" barrel but has the long gas system with the sights positioned as on a 20" barrel).
Set your sights with the rear set to 300 yards with windage set to dead center and the front bottomed out. Shoot 1–2 shots; if they're not in the black on the target, move as follows: if centered but high, elevate front sight about 2 clicks and shoot again. If you went too far and the group is a bit low, drop the front sight back down a click. Windage...move the rear sight in the direction your group needs to go 1 click at a time. These are usually 1/4" MOA adjustments (each click) so remember that the effect is multiplied downrange. Once you have a 3-shot group in or close to the tiny black silhouette, you are good from 25-300 yards (or meters, within close tolerances). Bear in mind that if you have a 25-yd./meter zero and you are shooting factory ammo or reloads to those velocities, you will be about 1.6" high at 50, 3.5" high at 100, about 4" high at 150 then the arc starts downward so that you are right on again at 300. This worked for my Del Ton kit with a military MaTech rear sight and saved me a bunch of ammo. You will be amazed at how accurate your AR is, even with iron sights! Learn to shoot iron sights then if you must, "graduate" to optics. I stood behind the firing line at Pendleton a couple years ago, watching boots practice ventilating 200-meter bulls...no optics there, as iron sights were good enough. If you don't reload, start! You'll shoot a lot more for the same amount of money. Good luck!