This is my first guide so please forgive me that it doesn't look 'fancy' but i thought i better keep it simple.

Elise is a very versatile champion. Mastering her both forms is a key to victory. In my opinion she performs best in mid lane but she can be decent at top or jungle depending on who you will face there. I don't recommend this build for top or jungle thought. But if you will decide to go top or jungle just change mana Seals to armor and change 21/0/9 to 21/9/0 picking up armor and health in defense tree. I tested this build several time and indeed it works well for Elise in my opinion.
I recommend leveling her W 1st because its hard to farm with her and W makes it easier and get Kage's Lucky Pick to help with income specially if your falling behind.

PROS
Her Q does % damage (if they have more hp you do more damage)
Has a stun (always useful)
Can catch running enemies with her E in spider form
Her E in spider form reveals nearby stealth players (or in brush)

CONS
Hard to farm with early lvls
No reliable escape tool
Need to master both forms and cooldowns


Switching forms at the right moment is important with Elise. You can do your standard rotation in Human form then in Spider form and back to Human form once you know cooldowns will be ready to finish them off.

For mid lane i suggest standard AP runes. Magic resist will help to sustain other AP champion and mana regen will help for you to sustain your Human form and pokes with your Q.
As i mentioned above if your going top or jungle i suggest to replace mana regen Seals with Armor and 21/9/0 masteries picking up Armor and Health in defense. This will help you face Healthy champions.

Standard AP mid masteries will help you to keep up with enemy damage early and late game.
I didn't pick up magic resist or armor from defensive tree because you spider form and items in the build will give you enough defenses against most champions.

I suggest to open with Boots and Health potions. This is pretty standard and helps to avoid pokes from other champions such as Orianna or Cassiopia. [*]Kage's Lucky Pick[*] will help you with income since its not easy to farm with Elise.
Now lets talk about your sustain:
Rylai's Crystal Scepter is great for your sustain HP AP and slow its great.
Wit's End makes your W more powerful and you do even more magic damage and get additional sustain in fights.
Abyssal Scepter is great for Elise because you fight in melee range most of the time. Helps to fight enemy magic resist and gives you further sustain.

At early lvl's focus on last hitting minions and when you get a chance switch to spider form to make farming slightly more easier. Unfortunately you will be able to clear minion waves easier/ faster only at around lvl 9 when your W is maxed and you got more Ability Power. Don't forget to poke your enemy with your Q or throw full E > W > Q combo.

In team fights i tend to poke as much as i can in Human form and when i switch to spider focus AP/AD carry once target is dead back to Human do your combo again and back to spider form to finish off. As i said switching forms is important to maximize your damage and secure kills. Don't be afraid to go all in on one target even if you die you will pick up a kill and its better trade then dying without any kills.