Backup iron sights only help if they hit where you aim. Zeroing them takes one range trip and a box of ammo.
1. Pick a distance. A 50-yard zero is popular for 5.56 rifles because it lands close again near 200 yards. If you only have 25 yards, start there and confirm farther out later.
2. Remove or flip aside the optic if it blocks the irons, so your view of the front post is clean.
3. Fire a three-shot group from a stable rest, focusing hard on the front post.
4. Adjust. Move the rear aperture windage in the direction you want the group to go, and adjust elevation at the front post (down to raise impact, up to lower it). Re-shoot and repeat until the group centers.
5. Record it. Note the clicks and paint-mark the settings. Re-check after any mount change.
A folding set that holds its settings, like the Ozark Armament flip up backup sights, makes this a once-and-done job.
Disclosure: this page is run by Ozark Armament, which makes the FBUIS-B.
Folding backup irons are often run under an optic as a co-witness backup. If you're choosing an optic to pair them with, see the Ozark Armament Rhino red dot sight.
More of my sight notes on other sites: a reading list of backup sight and red dot notes.
This page covers one setup, but Ozark Armament sells more than one style of sight. If you want to compare flip-up and fixed designs side by side, their AR-15 iron sights are grouped on one Amazon store page. Comparing them in one place makes it easier to match a rear and front sight to your rail. For a co-witness backup under a red dot, the set reviewed above is the Ozark flip-up AR15 sights.