Answer:
You will be sighting in for a specific range, say, 100 yds. You may need to start at a shorter range- 50 or even 25 yards to get the rifle on the target. Shoot from a rest- preferably a bench/ sandbags, or some type of solid support.
Adjust the sights as needed- on most rifles, the rear sight will be raised to raise the strike of the bullet. It is moved in the OPPOSITE direction of the bullet strike to adjust windage (left/right). I did say MOST- some rifles are different.
Fire 3-5 shots, note where the bullets strike the target, and adjust sights. Once you are on target, move out to 100 yards, adjust. Some military rifles are sighted in for 200-250 yards. For closer shots, aim lower, for further, aim higher.