Being a sniper in ArmA games is not easy and requires patience and tactical approach.
The most important part is to keep a distance from the battlefield. Best to find yourself a hill or a mountain from which you will have a good view on what's going down on the battlefield, and then find a spot in the bushes and lie down. Make sure you wait a moment, if you ran for a period of time. Running makes you tired, and fatigue has a great impact on your accuracy.
After finding a good position, you will not have to find some enemies to kill. Use the rangefinder by pressing B and search for a target.
Once you've found one, hover the red dot in the center of your rangefinder vision. On the right side you will notice the distance in which you are away from that target. Remember that number and put away your rangefinders.
Now pick up your sniper rifle and you will have to calibrate the zeroing on that weapon, so that the bullet travels the desired distance. So for example - if your targer is about 1900 meters away from you, then you have to set the zeroing for the same distance. You do this by pressing the Page Up key (goes up by 100m) and Page Down (goes down by 100m). You can also zoom in with your scope by pressing the + key on your numpad.
After calibration, you will now have to make an accurate shot. If there's a great distance between you and your target, the weapon will slightly swing, even if you're lying on the ground. You will then have to hold your breath, and you do that by holding down the right mouse button. Aim the red cross in the middle of your scope at your target and shoot when you're ready.
Remember - if you're shooting at a long distance, the bullet can travel up to two or three seconds, so do not fire a second shot too soon.