So, you pick up the reveared sniper rifle and try to use it. Maybe you succeed, maybe you don't. Hera are some tips to make sniping easier, and more rewarding:
I reccomend keeping zoomed and and watching for movement when sniping. If you do not see any movement, zoom in to the first level and scan the area. MAKE SURE TO KEEP MOVING UNLESS YOU HAVE A TARGET. Only zoom into the second level if the target is far enough that he won't run out of your Field of Vision.
Whenever you face an enemy, you are faced with a choice: Go for the headshot or play it safe with body shots. If your target has no shields, use your common sense and hit the body. however, if the target is shielded, it is a different story. I beleive you should go with your gut instinct. Do what feels right. If you are confident enough, take the headshot. On many accounts i have taken a last-second headshot and gotten the kill. However, if you are sure you'll miss, it is better to save your ammo and not give away your position. If the target is in the open, and you have enough ammo, and he is aware of you, it's best to go for the body. If he's stationary, or even unaware, get the headshot. Again, go with your gut feeling.
Practice, practice, practice:
Keep practicing your sniping. Especially close-range. I used to be unable to hit a wall at close range. now i geat point-blank headshots. Sniping playlists are a great way to practice because they force you to learn to use the sniper in any situation.
Learn to use both snipers. Know each sniper's advantages and disadvantages. The Human sniper lets you squeeze off 4 shots in rapid succession, but needs a reload. A well-wielded covenant sniper can squeeze off either 3 shots rapidly before overheating, or, if you aren't as fast on the trigger, many more.
Again, there are no concrete rules. Do what feels right. Good luck!