In the previous video, Jeff explained how to sight in an optic on your rifle. If your rifle is an AR-15, chances are it came with - or can easily be fitted with - mechanical "iron" sights for backup, in case your scope or red dot breaks. Just like an optic, iron sights need to be sighted in, and the first step is to make sure they're installed correctly before you hit the range. It's best to choose front and rear backup sights made by the same manufacturer. Mount them with as much distance between them as possible. Backup sights are meant to be used only in an emergency, so zero them for 25 yards. Then work out where to hold for targets at 50, 75, and 100 yards. Start by adjusting the windage at the 10-yard line. Get it dialed in as precisely as possible, THEN move back to 25 yards. Shoot from a stable platform like a bipod, shooting rest, sandbags, or even your range bag to eliminate as much unwanted movement as possible. Zeroed iron sights probably won't need much if any adjustment. It's good to check them and confirm the zero periodically, especially if you switch to different ammunition. You can also "co-witness" your iron sights with the red dot - view them through the red dot's lens and synchronize them to its dot to sight in them in.
