Mastering the sniper rifle, the essential tool for completing many objectives on higher difficulty levels, can be a true challenge. Once you aim through the scope you can shoot anytime you want, except if you take anything higher than Cadet simply shooting will be not enough.
First, you need to check the distance to your target - to do this, use the binoculars and mark your target - you will see distance in meters above the target's head. Now, you have to adjust the scope by using the Q and E keys (left / right direction when on pad) (1). One click adjusts the scope by 100 meters. If your target is, let's say, 120 meters away, you need to set your scope to 100 meters and compensate for the bullet trajectory by aiming a bit above your intended impact point. This works both ways - you can also set your scope to 200 meters and compensate by aiming a bit more below your intended impact point. Note however, that it will be more difficult to assess the right compensation when your regulations are not optimal. If possible, you can also change your position to have the distance match an exact number.
When playing on Sniper Elite or Authentic difficulty, you'll also need to think of the wind (2). At 100 meters the bullet should hit exactly the spot pointed at by the arrow above the scope. A relatively easy way (at least for when shooting at a static target) is to aim the wind arrow at your target and then add vertical compensation by raising your aim. You can also try to position yourself so that the find blows in your face or in your back - this will save you from having to add lateral compensation. Remember that wind power can change when you're aiming, so try to aim without delays.
In the right lower corner (3) the game displays your current stance. You can kneel or lie down for a more stable aim. You can also use focus (space / right button when on pad) to stabilize the scope. Note, however, that you cannot focus for long or after you've been running, when your heart rate (4) is high - just wait a bit for it to calm down.
One more thing you need to remember is that a shot from a non-suppressed weapon is heard across a wide area, which may alarm the guards. If you'd rather avoid trouble in an area full of enemies, try to use suppressed ammo (the type of ammo you're currently using is shown in the right lower corner (5)). Suppressed ammo deals less damage, and can fail to finish off an opponent who was has already seen you. Standard ammo penetrates wooden objects.
Binoculars enable you to learn the distance to your target. Once you've marked it, you will see the target even from afar, and if you mark it long enough, you will see additional information on the right: possessed items, type of enemy, possessed weapon and secrets (assuming the target has any).