From my experience, it's pretty painful. She's mage tier when it comes to mana whoring, but doesn't get the mage tier damage to back it up unless you're retardedly fed. It's one of those things you can do, but no one would recommend it to you as a viable option.
Laning
Lane patterns are pretty straightfoward.
- Use your human form Q to wittle down their hp, and EWQ combo whenever you can land a cocoon.
- Care not to just toss out your w willy nilly because it'll just shove the lane in exchange for sub-par damage.
- Shoving as Elise not super great since you'll be behind the minions you can actually rappel to in order to escape ganks.
- Once they're low, spider form and go in for the kill, less than 30-35%hp recommended (with their hexdrinker down).
- If you feel like you're going to get camped by the jungler, try to save your cocoon for disengages (when they gank you cocoon the more mobile one)
- The closest meta example I can think of to Elise, kit and pattern-wise, is as an AP version of Jayce.
Items
I would recommend double doran's into seeker's. You can rush seeker's first if you really need to for the armor, but double doran's feels really strong on Elise because it addresses most of her early game issues quickly (hp for beefiness, ap, mana regen). Seeker's into Zhonya's is against AD lanes, of course. Against AP tops, you have more flexibility. You can go into your core items (Haunting Guise/Magic pen boots) or you can pick up Catalyst and go into RoA if you're having some trouble.
After you get your first hard damage item (or core stuff), you should start getting tanky. Typical stuff like Locket, Banshees, Sunfire, Randuins. You know the works.
Abilities
Start Q. Max Q ->-W->E.
Whether or not you hold off putting points in R is up to you. Your spider form will have better sustain damage, but it'll also shove the wave faster when you're in spider form farming. Personally I'd do it, but I can understand why someone would rather get better q-w dmg.