You see that enemy over there? Ok good, so basically, put four fingers on the four QWER buttons. First, press Q. Is the Q out? Great! Now, with complete precision, mash every key mentioned above at least 5 times. Congratulations, you just learned leblanc.
But really, leblanc is pretty easy to learn. DFG is essential (preferably as the first item unless you're uncomfortable with that). Get level 2 as soon as possible, then use the Q > W combo whenever you can, you won't need to do it much before the enemy falls back. Add a nifty E if you think you can go in for the kill.
At level 6, the usual combo goes as Q R W E, keeping in mind that R (which will mimic the Q) will proc the Q, and the W will proc the R, which is insane burst as it is.
You'd be surprised how many times you'll mimic the wrong ability in your first games, you'll need to practice keeping track of which ability is up for mimicking. Same goes for remembering that when you use your W, you have a fairly long-ish time to blink back, you might have to keep an eye on the buffs down below to get used to how long it actually is. Mimicking W will also confuse you the first times to get used to which one you want to blink back to. These are basically the only mistakes you can make as lb.
Practice a couple times, you'll actually have a hard time, then 4 to 5 games later, watch yourself one shotting everything that comes in your way.