First off, I want to make this clear. All guides are SITUATIONAL. That means that, even if a guide works 90% of the time, there is still that 10% where It would be better to use something else. With this in mind, this guide is a ABILITY POWER DAMAGE GUIDE for the champion Leblanc. Still with me? Then let's begin.

Flash and Ignite

This is the combo I will take in most situations. Leblanc is a powerful DPS, and has innately high mobility. However, as Flash is a very strong option in most situations, allowing for massive outplay potential and instant mobility. It is a great tool in your toolbox, but not always the best. If this is a bad option, I suggest Teleport as a replacement.
Ignite, on the other hand, is mostly for dealing with a tank. The tank is, by nature, difficult to kill, and ignite gives an answer to that. Grievous Wounds is more powerful than most players in the lower tiers believe it to be, as it neutralizes a large amount of their sustain. It also gives Leblanc that little bit of extra damage to kill the enemy. However, against a damage-heavy team, it would be a terrible option. If this is the case, bring Exhaust.

In all games, items are all situation-based. If you're sure that getting Banshee's Veil is a better choice, go for it. But in most situations, it's best to stick to what you know. So let's talk about that, shall we?

First Buy - Doran's Ring and 2 Health Pots
In any given game as Leblanc, your job is burst AP. And Leblanc is a squish. Ring for AP burst, Health Pots for squish. simple, right?

Damage
All the items in the Damage catagory (shown above) are for, well, damage. With a DPS role in mind, this is ESSENTIAL for success. Pick a few, stick with them. Simple.

Defence
There will be some instances where you need more survivability. However, this does not mean that we build tank items - Leblanc isn't a tank. So we build tanky DPS items. There are very few good options for this, but enough that we can still live long enough to do our job.

Boots
What build would be complete without boots? With this, we just want to maximize the damage output for that situation. If the enemy team is building MR to counter you, Sorcerer's Shoes. If there is heavy CC on the other team, go for Murc Treads. Otherwise, Give yourself extra mobility with the Boots of Mobility.

Apart from items, this is the most situational part of the guide. Ability order is largely based on your opponents, but in most cases, this is what I go for.

Shatter Orb (Q)
This is your main damaging ability, and as such, your best go-to in most situations. However, as it is purely damage and has no utility, it's a terrible first choice. Max this quickly, first if you can, but not if the situation says otherwise.

Distortion (W)
Pick this first EVERY TIME. This ability is the equivalent of a lower-cooldown, damaging flash that you can return to if you so choose. It's AoE, so farming is a simple task with it, and the escape potential is undeniable. DO NOT UNDERESTIMATE THE POWER OF THIS ABILITY.

Ethereal Chains (E)
This is where your burst becomes sustain. Stunning an enemy to get your other spells off cooldown is a great way to spend time instead of chasing the enemy into what will likely be a trap, and it allows you to absolutely DESTROY any non-tank you come across, and a few low-key tanks, too. The only issue is that you have better, higher-priority abilities that you need to get before this one. Max this last, as the stun is all you really ever use it for.

Mimic (R)
Your ult. Max at every chance, as it reduces the cooldowns and increases damage dealt. Many of your combos will rely on your ult being up, though the basic ones don't. R-Q is your big bad DPS route. Use it when you need that little bit of extra damage. R-W is largely for escaping the enemy team after a team fight. Use it for extra mobility. R-E is almost never used, because your normal E does the job fine on its own. R-R is a special case. It sends out a copy of yourself, which then looks for an enemy and casts a fake spell on them - that spell being whichever one you cast last.

Mirror Image (Passive)
This is by far LeBlanc's least useful skill. However, by no means is it useless. Use this to confuse your opponent and make your escape.

Leblanc is a DPS champion, meaning that your build should revolve around damage. This being said, Domination is the first choice for Leblanc.

Following that same train of thought, Precision is a powerful tool to capitalize on the damage you deal. Use these runes to gain maximum effectiveness from your damage.

All of the above are things to know before the game even starts. But what about once you get into a game? From here onward, I will explain how to use Leblanc in different situations, and how to put yourself into good ones. Keep in mind, however, that these are situations. Unless you end up in one of the situations listed, DO NOT use that situation's action, as it will usually get you killed.

Sit. #1: Enemy is running away at low health
If an enemy tries to run, and they're at low health, one of two things is happening: They want to live and are looking to recall, OR it's a bait to lead you into certain death. The difference between the two is difficult to explain, but if you are able to kill with one hit, do so. If not, let them run.

Sit. #2: You are about to die, and your team is close
Run toward your team and signal for help or danger. A ping to your location is a great way to get your team's attention, and if your enemies are set on killing you, you can lead them into all sorts of danger. In this situation, your life doesn't matter - the enemy's slaughter does.

Sit. #3: About to die and distortion/flash is up
Run to the jungle and use your distortion/flash to jump a wall. Even if you only delay them, it will give you that much more time to plan out your next course of action. A wall is an easy barrier, and if you think they can follow you over it, shoot a Ethereal Chain that way. They can't see it coming, after all!