Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
The items are based largely around lifesteal, but also serve the other aspects Caitlyn requires; damage, attack speed, and movement speed. Crits are less important, but the Phantom Dancer at the end provides a great end-game boost to your already high damage.
The Doran's Blade is advisable if you are mid laning, because it provides the health and damage needed to look after yourself, whilst slightly increasing your health with each shot. If you are side laning, it is not as important, and you could start directly with a Vampiric Scepter.
I choose Boots of Mobility because Caitlyn needs the speed to keep up with fleeing enenimies, helped by the Frozen Mallet, and to escape those chasing you down. Her cooldowns are low enough not to warrant Ionian Boots and she does not need the relatively small effect of Berserker's Greaves.
The rest of the items are based on lifesteal/survivability and damage, so are quite self explanatory, while the Stark's also assists your teammates to kill team targets as well.
I take runes of armour penetration and attack speed, meaning it's great for early game harassment and for late game breaking through of a little bit of additional armour they might have bought, plus since the items do not provide a huge amount of attack speed, gives a small boost in that area.
My masteries are a bit off the standard 21-0-9, but mostly because Caitlyn doesn't so much need the Utilities as she does the damage and midding ability. That said, with this build, both laning Caitlyn and mid Caitlyn are very viable and great scores and games can be achieved with either.
The skill sequence I have chosen because of mobility. The Piltover Peacemaker is one of my favourite spells of the game, because of the range, damage and penetration of an entire lane of minions, and needs to be levelled up as a priority. The 90 Caliber Net is great as a slowing ability, chasing ability, and escape ability, and the reason for levelling it over the Trap is for the cooldown reduction, to give that extra jump. The Trap is also great for stopping gankers waiting in the brush, and stops people in the lane in their tracks to receive a Peacemaker to the face. And the Ace in the Hole is just an amazing ultimate, requiring some thought to when you can shoot, and should be levelled ASAP.
Clarity and Flash may be an odd combination for a ranged DPS, but it works well; because of the mobility Flash provides, either escaping or getting in position for the Ace in the Hole is very simple and will help keep your kills high and deaths low. The clarity is pretty essential with this build, because as Caitlyn, you need to use your spells a lot, and without any mana items or runes, you don't want to find yourself in a situation where you haven't the mana for a convenient net or finishing Ace in the Hole. You will use it throughout the game, yet will rarely find yourself completely without mana and with a long cooldown remaining, so it's always useful.
Mobility, or, Getting Away!
If you find yourself in a tight spot that you don't think you'll win, be it chased down after a team fight or a nasty gank early game, try and get to a thin wall to Net through, or failing that, Net backwards towards a tower and away from the enemy and immediately place a trap where you land, the slow should be enough to allow the trap to set and stun the pursuer. Then of course, Flash if needed, but it should be saved for offensive action rather than defensive ideally.
This build has served me very well recently, to devastating effect on some occasions, providing amazing powers of DPS against most characters, and also allowing great 1v1 ability and survivability with the lifesteal. The lifesteal works along with the passive Headshot, allowing a shot which gives a great boost to health lost. The items give high damage, amazing lifesteal, and great mobility, which is pretty important on Caitlyn for her DPS and supporty roles with the Trap and Net abilities to help get her or her teammates those important kills.
Thanks for reading.