After those 3, get either Rylai, Abyssal or Zhonya, whichever is better for the game. You'll want to end up with all three. Although you'll only have about 150AD/AP around mid game, your damage is insane and you can take a bit of a beating. The cutlass gives you the slow to stick to your target, and the sustain is quite useful with that 140% AS
R > Q > W > E
Spam that Q on the enemy every time it's off cooldown, and when they're about half, from human form. W > Q > E (or E > W > Q) then go spider form and W > Q > E when they run/flash.
It takes some practice to get used to her, but when you do, she's actually really strong.
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You failed to learn one of English's two one-letter words. That's just special.