This guide is going to be rather boring, as the Leblanc has only 1 role and lane, and her items are pretty obvious.
The playing style of Leblanc is last hitting with autos, to get mana from your ring(s), before q, w harass. When they're low enough, you unleash your full q, r, w, e combo. Since Leblanc is so squishy, you want to use your w/r to escape prematurely, so you don't get caught by cc, and then you'll have enough hp to keep laning. If you have a teammate(s) with you, and you're trying to kill a tanky enemy, use a e/r double snare, and let your team do the damage while you hold them.

Your core items are haunting guise, mejai's and flask. The reason being you need the hp and pen from guise, and because Leblanc, is like the opposite of a carry (bad endgame), you need to have the mejai's you racked up early when you're really op. Flask covers your hp needs, because you wouldn't get rod or spell vamp, and you're not good at killing wraiths for hp back. The rings cover your mana/health needs, but you might need more mana is you're last-hitting with spells, so you can grab a chalice, to be sold later.
Your next items are deathfire, for cdr and really effective with your combo; void staff, because you're a burst champ; zhonya's, because you have no other defense (be sure to get 15 stacks on seeker's first); and rabadons for obvious reasons.
Your boots are sorcerer's, and your enchantment must be homeguard, because you're always low on hp and mana when you recall, and you're slow...this'll save you a lot of time.
I see a lot of Leblanc builds include Liandry's...which is actually really stupid, you do your q, apply the proc for 2 seconds, do the rest of your spells, and boom, you've applied the proc for less than 3 seconds...liandry's is for spam casters only.

Of course, you max out your q first (it's like maxing out your ult, as you use your ult on q most often), then w (farming, in conjunction with your to harass), then e, because you only need it for cc.

The offensive tree is pretty obvious, you can swap out a point of havoc for cdr. Lots of other guides spend the remaining 9 points on utility. When you play Leblanc, lane presence is more important than harassing or cs (yeah, they're pretty important, but the way to counter a Leblanc is by making her recall and blowing all her mana). And since I don't need that much mana and cdr on my summoner spells, I got with the standard 9 point mid defense tree (non mid defense would be 3 points on armor, 1 on magic resist).

Some people like going full ap/mr pen on burst casters, but different rune types have different values (e.g: armor seals are better than armor glyphs). So I do the traditional armor seal mr glyphs, with hp quints, because my final build has no health at all, and a combination of ap/mr pen marks.