Put on ghost, cold blooded and dead silence. On the sniper, put on a silencer. Unlike the other cods where silencers made it really hard to get kills, this game doesn't do that. The silencer on the sniper rifle takes away nearly all the sound. People are oblivious to you shooting at them. I've had many people never turn around even if I was shooting right behind them. It is much more quiet than a silenced SMG.
Find a good sniping spot that is far away and up high. With ghost on, and coldblooded, it will be very hard to spot you. Another tip would be to have the scope up a lot so if someone watches the killcam, they can't find your location.