Really, the secret to sniping is to be Me. LOL, just kidding.
In my honest opinion, the trick to sniping is finding out just how you use a sniper.
Do you hit them from across a map? Do you use it like a shotgun (no-scope/melee)? Do you quick scope? Do you only no-scope? Or do you use it to cover an objective from not that far away (think the tower on the Pit...sniping to top platform or rocket hallway)?
Here is a hint: If you are using it as a no-scope/melee weapon, keep the sensitivity around 3-4. Across a map like Valhalla you want it around 1-2. Quick scopes should be around 6-7 or higher. A normal distance sniper should keep it around 4-6...try 4 for starters.
Then it becomes being good at what you want it for. I don't pick up a sniper often on bigger maps, mainly because I am not too good at large distances and I find that the BR is good at stopping snipers on big maps. I will use it alot on Epilogue and smaller maps because I am good at swipe sniping and quick scopes. Find out your strong suit and develop it. Start off asking for the sniper in Social Slayer and playing custom sniper games. Then work on to ranked slayer, and only after you feel strong with it do you go into Team Snipers.
Keep that in mind, and don't be afraid to change your sensitivity depending on what map you are playing. For close range, small maps, keep the sensitivity higher than normal (as long as your BR is still accurate...). On long distance maps, tone the sensitivity down some to make it more steady. It will come in time, trust me. Halo requires accuracy and patience, so don't get aggravated if at first you don't get the sniping down. Just work on it and it will come to you.