First off, find your own style. We can tell you what to do and what gun to use... but we are just telling you what works for us. Find your comfort zone.
I am a less than average sniper, myself. I play HCTDM and have just never had the patience for sniping. I got Diamond on my SMGs/LMGs/ARs and decided to try a little sniping just to mix it up.
Whats worked for me so far is
1)Keep moving. Sure, if you can find a good spot stick with it for a while, but what works best is to displace. Get a few kills from a spot, move to another one. Sometimes I will go to a spot overlooking where I just was and pick off the guys coming to hunt me down. If you stay in one place too long you are a sitting duck.
2) Don't get Scope "tunnel-vision"- This game moves so fast that if you look down the scope too much you will miss opportunities. Sometimes staying out of scope and zooming in once you spot an enemy can be more effective and enable you to get a clear sight picture.
3) Awareness and map knowledge- Learn your sight lines. The maps are small but there are a lot of maps that have clear sight lines to high traffic areas. Also, its key to communicate with teammates or at least have a sense of the action going on around you. If you know where your team is spawning you can set up accordingly, and if you keep track of where teammates are dying you can anticipate shots. Sometimes you can set up in a high traffic area and get a kill when someone appears, but I get most of my best shots by anticipating the enemy based on where I know my team is and where there is action or movement on the map.
4)Carry an effective Sidearm- Use a pistol that you can swap to quickly. Fast Hands is a great Sniper tool and has gotten me out of sticky situations when someone sneaks up on me.
Good luck and have fun!
Start playing using high sensitivity. Start with 7 and move up as you go. I snipe on 14. Control freeks on the right stick helps a lot. Start with the DSR and play slow. Camp on obscure areas and take your time to aim. Don't get too excited and reveal your position. Hardscope. When you get a hang of hardscoping, your target acquisition will get faster in time and you will be able to quickscope.