Although I am a skilled LeBlanc player, I am not the best LeBlanc player, so I'm going to tell my personal knowledge in a simple way and try not to confuse anyone. If you feel I am not a good source, feel free to get advice from others.
Sometimes there will be scary math, disclaimers, etc. -Read At YOUR Own Risk-
Disclaimer: LeBlanc is a very difficult champion to pick up, learn, master, etc. Play at your own risk.
In this guide I will go over LeBlanc's Runes, Masteries, Items, etc.
I use Faker's Runes, but I swapped out a few for my personal preference
*Magic Pen Reds
*Magic Resist Blues
The reason i don't like Faker's runes is because his blues are magic pen, like his reds. I like the extra magic resist because more than likely i am going to be playing against an ap midlaner.
I also use Faker's Masteries, but swap a few out for my personal preference.
I go 12 18 0 Masteries with LeBlanc.
Cunning: Wanderer-->Assassin-->Merciless-->Dangerous game-->Precision-->Thnderlord's decree.
Ferocity: Sorcery-->Double Edged Sword--> Natural talent-->Oppressor
The other keystone you can also run is Stormraider's Surge. I personally don't like this mastery because LeBlanc has gap closers and movement built into her kit, so I see this mastery not useful.
Faker runs Runic Affinity, I run Assassin. WHY? Faker prefers the extra time he can use with his buff to pick up a kill, poke more often, etc. I prefer the extra damage. Some people use Secret Stash to regenerate health faster. It is all preference. Whatever you feel best with, use.
Items are as follows:
Start: Doran's Ring-->2 Health Potions-->Ward
Core: Morellonomicon-->Scorcerer's shoes.
Options if behind: Abyssal scepter OR Zhonya's Hourglass
Final Options: Rabadon's Deathcap-->Luden's echo--> Lick Bane-->Hextech GLP-800.
These are the items I use. Some of the items are only necessary if you are behind.
Disclaimer: Just because you are fed does not mean stop farming.
Usually take Flash+Ignite. Flash as a basic need for most champions, can be used as an engage, disengage, etc. Ignite to secure kills, or to add more damage as your pulling off your combos.
Teleport: use this if you are going to play a passive game against a counter, objective based team, etc.
Cleanse, Barrier: Use these to clear, shield yourself against any stuns, ignites,roots, etc.
Don't use any other spells, unless you are on ARAM, use Clarity in that case.
Those are the basic steps to learn, and master LeBlanc. Anything after this point is strictly tips for succeeding with LeBlanc.
Pros / Cons
*Very dominant laning phase. *Very squishy.
*Lots of burst damage. *Mana problems early game.
*Lots of roaming potential. *Hard to play properly.
*Lots of Combos; options. *cc = Dead.
*Extremely Mobile. *Hard to carry.
Creeping/ CSing/ Farming.
*LeBlanc has a very easy, smooth auto attack antimation, making it easier to farm with her.
*Q can be use to secure farm, such as super minions, cannon minions, etc.
* Farming under tower:
Melee minions: 2 tower autos, 1 LB auto.
Caster Minions: 1 LB auto, 1 tower auto, 1 LB auto.
Team Fights, Mid game.
Use your roam potential to pick up kills for other lanes, putting them ahead.
Let your team mates start the fight, and you follow up. Focus one opponent , preferably carries, supports, etc. to pull off lots of single-target damage. Use your mobility to your advantage. Try to steal kills. REMEMBER TO CS/FARM/CREEP!
Note: The following text is stated as if you already know what her abilities do, read at your own risk.
Your most basic combo is: Q-->W-->E, or E-->Q-->W, or W-->Q-->E. - There are many variations of this pre Level 6 combo. Each is used for your personal preference. Although the first combo listed does the most damage, so I will go in depth about this combo. Use your q before any of your follow up abilities to pull off more damage and not miss out on a potential kill. Use w to gapclose, making it easier to land e.
Your level 6 combos are as such listed below:
Q-->R-->W-->E This is your highest damage combo. Use this combo as your most used combo.
W-->R-->E-->Q This is your gapcloser combo. Use this combo to kill a straggling opponent, tower dive, etc. Use ignite to secure if needed.
E-->R-->Q-->W This is your cc combo. This combo will proc 3 seconds of root allowing for team mates, or yourself to pull off more damage onto an enemy.
A very good combo to use to escape enemies is called the "Bushie Bush".
In this combo, start off by leading in with w, then walk some distance, (prefer to go to another bush) then go back to your w location. This combo throws off many players. This combo is great for escapes, deception, picking off a target, etc.