Try these steps. You will have to experiment a bit to find out what exactly works best, but here are some good hints for you.
1) Natural alignment. Stand facing your target head on. If you are right handed, make sure your right shoulder is lined up with your target, vice versa for left handed people.
2) Lay down ( i'm assuming you will use the prone position ). Make sure that right shoulder is still lined up with your target. Place the rifle butt snuggly into your shoulder, put your cheek right up on the butt and pretend it's like your pillow. Make sure you are a few inches back from your scope for good eye relief.
3) Put your left arm comfortably out to the left of your rifle on the ground. Wrap your right hand around the stock, slightly behind the trigger guard.
4) Line up your crosshairs on the target and steady your breathing. Breath in deep, hold for a sec and let it out. Breath in again, hold for a sec and let it out. Breath in again and slowly exhale, roughly half way through exhaling your body will fell very steady and calm, this is where you will fire your shot.
5) Squeeze the trigger very slowly and fire. Keep those crosshairs lined up with the target, and even after firing maintain the point of aim. This is called follow through.
If you do everything correctly, there should be no movement of your rifle while aiming, firing and after firing. Getting into a comfortable shooting position requires practice, so does breathing properly, and proper trigger control also. Get lots of practice on the range to see what works best for you. Good luck.