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.
For runes, I typically use armor pen, mana regen, and attack speed. You may find flat health quints will help with early survivability, but normally you should be fine with armor pen quints as well.
My masteries are set up in a 21/0/9 build, with 2 points into Offensive Mastery to help with those minion kills, and a point into Utility Master for extended duration on my buffs. The longer you have those buffs, the more dangerous you are. Other than those slight differences, the masteries are typical for pretty much any physical ranged dps carry.
I start out with a doran's blade and a health potion. The extra health from dorans along with the lifesteal and damage greatly increases your usefulness. My first time back to base, I either have a kill or 2, or have farmed enough minions to buy another dorans and my berserker greaves. Third time back, zeal or phantom dancer, depending on how well I am doing. Fourth, BF sword for massive damage. Then a black cleaver for crazy attack speed, armor pen, and damage output. Afterwards, I buy items depending on what the game looks like it is headed. If it is going to be long, I normally grab a defensive item, then a bloodthirster or infinity edge. If I think it will end soon, I go straight for damage items. *Madred's Bloodrazor may be a good item at some point in the game if the enemy team is stacking health. I would suggest getting it after your black cleaver*
This is perhaps one of the greatest harass passives in the game. After every 7 shots, your next shot deals bonus damage (150% to a champion, 250% to a minion). This passive is doubled if you are firing from within the brush (ex. 2 shots in the brush count as 4 shots outside of it). A tip for getting the passive charged quickly is 3 shots in the brush and one outside of it. Then make your way to the nearest champion and fire. If they back off, attempt to chase them outside the range of experience. If you fire on a champion, back up and recharge your passive again. However, headshot is also an amazing farming tool. Early game you can kill minions that are at half health if you have headshot charged. Just remember to use it wisely. Using it to farm gold is great, but if you do 100-200 damage with a single shot at level 3 or so, you have an amazing harass there.
Starting out with Piltover Peacemaker lets me have a great harass as well as easy minion farming starting at level 1. Careful not to spam it however. I see far too many Caitlyns spam the peacemaker and end up with no mana during fights to be useful. Level this one up whenever you can.
Getting the Yordle Snap Trap at level 2 allows me to be ready for any gank. The best places for those are in bushes that enemies normally hide in to gank, such as the one near dragon, near baron, the 2 bushes next to mid, and the bushes just behind your tower. Since they are easy to see, throwing them in the middle of the lane is [usually] useless, because they can just walk on them and walk away, and the trap is gone without anything good coming out of it. I say usually because if I am laning with someone with a taunt or heavy slows (nunu, shen, rammus, etc) then setting traps where they will go to escape is a great idea. Only put one level into that until late game, since the damage is minimal and the snare lasts the same amount for each level.
Next is the 90 caliber net. This is one of the best escapes in the game, as it lets Caitlyn have a free flash that also slows any enemies it hits. It can also be used to help chase down enemies, allowing you to throw your net behind yourself to jump a bit further ahead. You may also jump over walls with this skill.
Last but not least, Ace in the Hole. This is a lock-on shot, never misses, and always deals massive damage to a single target at a huge range. However, allies are also able to jump in front of the shot to block it. The best times to use this are when there is a single low target without allies nearby, or a few targets that will all die if they attempt to intercept it. The other good time is when the enemy team is running from a team fight. If any targets are low, lock on to them. Normally, they all are too distracted with running away to help their low ally, granting a free kill.
I don't believe there are any bad summoner spells... I just believe some are better than others. Here is a list of what I feel to be the most useful to the least useful on Caitlyn.
1: Ghost- Almost a must-have for Caitlyn. Being able to run to/from targets is amazing, letting you chase for that easy kill, or escape potential death.
2: Flash- Great utility skill. Jumping to the perfect position to or from targets is great, and you can never have too much survivability.
Those are the 2 spells I take as Caitlyn. Here are the others...
3: Exhaust- Great for 1v1 fights. they deal less damage and are slowed for the entire duration.
4: Ignite- Good damage skill. Allows you to finish off targets (if your ultimate is down or you don't have enough damage with it alone). However, you are also a sniper. Being close enough to use this skill is dangerous in team fights especially. Use it wisely.
5: Teleport- Lets you get around the battlefield. Good for holding your lane (yay laning phase)
6: Cleanse- Clears yourself of debuffs. Nice escape mechanism.
7: Heal- Useful if you are low on health, or want to trick your opponent into thinking they can kill you.
8: Clarity- Want to spam your Peacemaker? Clarity is nice. Not much use once midgame begins however.
9: Clairvoyance- Good scouting tool. Better on a support however
10: Fortify- Enemy caster minions giving your towers a problem? Fortify it! Pretty useless on Caitlyn.
11: Smite- Not the greatest spell for Caitlyn, but helps clear minions and buffs faster. Caitlyn cannot jungle however.
12: Rally- More damage. Yay. Useless after the begining of the match.
13: Revive- You shouldn't die. Revive is a babysitter for people who die too much. STOP DYING AND YOU WONT NEED REVIVE. I hate revive.
Lets talk about how Caitlyn plays. I explained her skills and how they work. Lets put them to action. Try to grab mid if you can, unless there is another carry on your team who needs mid. If you can't mid, TRY YOUR BEST TO LANE WITH A TANK. You can do enough damage to piss anyone off on the other team, you just need a partner to soak it up for you too.
Harass enemy champions with your peacemaker IF YOU HAVE A GOOD SHOT. Don't waste it by trying to aim for them and hitting nothing. If you dont have a clear shot, hit the minions with it. Put snap traps in the bushes where ganks will come from and save your net for an escape. Get as many kills as you can, and ask your laning partner to back off if you are going to score a kill with your ultimate.
Team fights are starting. You should have your phantom dancer by now, maybe your black cleaver. focus the enemy squishies and throw your peacemaker out for AoE damage. Snipe anyone low with your ult.
Items should be complete/close to it. play similar to mid game, focusing squishies. Stay in the back of your team though. You dont want to be caught in the middle of the fray.
Victory. Enough said.