First of all this is my first Champion-Guide, so don't hate me too much.
Soo..LeBlanc. She's probably the most damage dealing singletarget burster ingame.
Thats why the enemy team will CC you and rape your face.
In this guide I'll try to teach you how to avoid getting targeted, still do lots of damage AND even support(a little).

Runes are pretty standard for casters.

9 Magic-Pen Marks
9 Mana/5 Seals
9 AP/Level Glyphs
3 Flat AP Quints

Magic-Pen Marks is pretty much the only thing to play on her.
Mana/5 Seals are not really necessary, but I like to give me some power in lane, especially Early-Game.
AP/Level Glyphs because they get stronger than Flat AP at Lvl 6, which is when your burst starts to shine.
Flat AP Quints because it makes you stronger early game.

Masteries are also nothing special.
9 / 0 / 21
Just take all the CDR and AP skills.
You could take 9 in defense, if you had a lot of problems in lane, buuuut you really want that Spell-Pen and your AP.
You shouldn't have too many problems in lane anyway.

Flash/Ignite is pretty standard for casters.
- escape (almost) EVERYTHING with W - R - Flash (or any comibantion of these)
- helps you, when you want to get away from your target but still need that little extra damage

Summoner Spells you COULD take:
- helps you to outfarm your enemy in lane and also for ganking / counter-ganking
- nice for escaping ( i prefer Flash for that, but take what feels better)
- nice for chasing, but in my opinion useless because you don't want to be near your enemy. Instead of Exhaust you just land an E-skill.
- can be usefull sometimes if you get CC'd in a teamfight and can't do anything. If you use your W skill wisely you shouldn't have any problems, but it can be a counterpick against a heavy CC team.

Summoner Spells you DON'T take:
Clairvoyance - you're not the supporter
Fortify - nope...not the tank
Smite - and not the jungler either
Clarity - with your mp5 runes you shouldn't have any mana problems and if you still do...see items
Heal - you dont need HP to fight enemies! Nuke from the distance
Revive - ...
Rally - ...

Skills:

(Passive: When you get below a certain percentage of your health you become stealthed shortly and after that an image of you is created which doesnt do anything but stand around and autoattack with no damage)
Q - singletarget nuke, marks taget for 3.5 sec. Target gets silenced when Mark is triggert by another of her abilities.
W - teleports you to the location you click
E - skillshot. when you hit the target you slow it and when the binding is up for a certain time target gets snared
R - mimics the last skill you used with more damage

Combos:

Q - W - standard harass
W - Q - R - (W) when your enemy just came back from base you really depress him by doing this.
Q - R - W(away) - E - great against melee fighters such as Tryndamere

I start off with one point in W at lvl 1.
If you're fighting for Golem or if you get ganked somewhere you can escape easily without wasting flash.
You could start with Q, but there isn't really a reason why you should.
You'll still be able to harass at Lvl 1, if you wish to.
After that two points in Q, to build up your Q - W combo.
Then one point in E.
After that it's just: R > Q > E > W This gives you some time to get get more combos off.
If you think your burst is high enough to nuke your opponent down in 1 or 2 seconds skill like this: R > Q > W > E

(Item builds:
1: The safe way
2: The happy medium
3: Glass cannon )

For items I always start off with a.
You could also start with boots and 3 pots, but you dont really need movement speed with LB because of your W-skill.

After that it's really opponent-dependent.
But most of the times awill do the job. It really gives you the sustainability you need for laning.
Also, if you think you're to squishy you can still buy another Doran's or two..

Then you should start building your, to be more mobile. (i.e. to escape ganks/your opponent on lane, go ganking)

If you had a strong start and you can getBEFORE 20 minutes ( ~15 mins are optimal...the earlier the better) then do it.
If ~25 minutes have passed and you still haven't got enough money for it, consider leaving the Catalyst for alater on.

Weather you have RoA or not, you really should buy boots next.
I preferbecause you just nuke one person down and then you cant really do anything.
(at least in early midgame..we'll come to end game later)
So just nuke with magicPen and then GET AWAY!
shouldn't need explanation. Just lots of AP. (still awesome after the nerf)
andif opponents build MagicRes.

If you don't do too great in midgame you can also build.
(Don't get me wrong Rylai's is a really great item. Just try it and you'll see if you 'need' it.)

Endgameagainst lots of AP andagaints lots of AD.

I don't really like Zhonya's, because if you need it, it means that you're not doing a good job of not getting focused.

Items you could pick:
Ionian Boots of Lucidity - not as much damage as with MagicPen, but you can get get off more combos
Mercury's Treds - against heavy AP / lots of CC
Deathfire Grasp - give you extra burst
Morello's Evil Tome - some AP and mana/5, if you have mana problems
If you have big problems you can also buy a pure defensive Item, but I like to have AP on almost every item. (not counting boots and banshee's)

Items you don't want to pick:
Archangle's Staff - you don't need that mana
Haunting Guise - just gets too weak too fast
Hextech Revolver - you only do damage to a single target so its not really helping
(Every AD / Hybryd item)

Last words go to Mejai's:
ONLY take Mejai's, when you're SURE, that you will dominate youre lane and be good overall against their team.
I consider myself a pretty good LeBlanc player and I almost never buy Mejai's.
But if it works out for you, it's a great item.

You should really take Mid with LeBlanc, because most of the opponents are no problems for you and you can farm a lot.

Opponents:

Probably the worst opponent (in Mid) you could possibly get is Morgana.
Her spellshield is up everytime you can get close to her with your W.
Try to fake attacks and when the shield is off you really attack her.
You'll have to skill W more then E in this case.
Cassiopeia can be difficult too sometimes. Just be sure to escape her poison.
Most of the other casters in Mid should be no problem at all for you.
Your burst is combined with a silence, so then cant nuke you when you try to kill them.
Also melee champs dont stand a chance against you, when you can escape their CCs.

Farming:

1. Rule: DON'T use abilities for farming, when your opponent is in lane, AT ALL.
That way you'll always have enough mana to fight. Your attack animation is very fast, so you shoudn't really have a problem lasthitting with autoattacks.
When your opponent is gone feel free to use Q, W or even W - R for farming.
Also while attacking your enemy you should be aware if you hit minions with your W-skill.
If you do, you'll be pushing even if you are just lasthitting with autoattacks.
Think twice before using W!

Laning:

Early game:
Lvl 1: Don't fight. Your strength lies in your combos and you have exactly none!
Lvl 2 / 3: Start harassing with your Q - W combo. It's not worth it to W and Q without hitting the enemy with your W! If you're in range for W - Q, you're in range for Q - W, which is 1000x better! Be aware of early ganks.
Lvl 4 / 5: You should now have your snare and be better prepared for ganks.
Lvl 6: When I get Lvl 6 I immediately try to Q - R - W, or if that doesn't work, get close with W and then Q - R. I do that to simply scare the sh** out of them. If you feel confident you can even try to kill them. Still be careful, because by now you should not have been to base and still be quite squishy.
Lvl 7 ... : You really should start to dominate your lane by now. When you land a kill and still have plenty health, try to gank Top/Bot.
If you have a few kills, it should be really easy to dominate Mid from now on!

If you are for some reason not doing good, try to stay passive and farm as much as possible.
Ask for a gank from your jungler or try to gank another lane to get back in the game.

Mid game:

Thats when you really start to shine!
You can nuke almost every champ down with your standard combos. Be aware of that and USE IT!
When you don't do anything in Mid game, you are no use to your team.
Build up your advantage against the enemies.

End game:
If you don't have 20-0-0 you should know that you now can't nuke everyone instantly.
You are still very easily CC-able, so be sure to W IN a fight, do everything you can and W OUT again. (within the 3.5 sec)
You'll be most likely nuking nobody anymore.
But if you have your Voidstaff and some magicPen you should be still able to do some damage.
Also be aware that if you E - Q an enemy, that the snare will trigger the mark from your Q.
This is VERY helpful in teamfights, because you silence AND snare an enemy, which is very powerful.
Be sure not to just nuke the most vulnerable target, but even use your Q - R on someone like Amumu to prevent the Ult.
You get the idea.
Your support side is, whats in the foreground now.

LeBlanc is great at really freaking enemies out.
The damage alone is too much for most of the players, but if you start to use your W skill wisely you'll really freak them out.
If you W over a wall and they flash over don't go crazy. Stay calm and press W again. Bam your back where you started and you can walk off with your trollface on.
Also great for escaping is W skill in one direction and if they follow you, you just W back and R / Flash in the other direction. Time with your passive for maximum troll.
But there are some people that know what to do against your W skill.
Lets say you play against Swain and you W in his face.
Most of the enemies will try to snare you where you are but if they were intelligent they'd put their CC where you are going back.
So be sure to get your combo off REALLY fast, so you can get your W - Q - R - (W) in about half of a second, before they even think about CCing you.
Get the silence off as fast as possible.

A great combo for towerdiving is W - Flash - (DAMAGE) - (W) . You can get really far with that combo and be exactly where you started when you're done.

Of course this is not the 'correct' way to play LeBlanc. This is just the way I play her and it works out 90% of the time.
If you notice any big mistakes please tell me and I will fix them.

Thanks for reading this guide!