You always take
first item, it helps with sustain during the first 3 levels. At level 1 take Q and just farm, if they're melee you can poke them out a little with it. Level 2 take W and use it on the river for ice element. Just keep farming slowly to let the lane push into you, then at level 3 you can engage if the minion wave is in your favor and chunk them for usually 50%-65% of their health.
You keep doing Ice element - wait for W to get back up again - e - q - w - q until they die basically.
At level 6 look for when they're next to those 4 small walls on both sides of midlane
https://www.slashskill.com/wp-content/uploads/2015/03/Screen01.png
These ones, and you do Ice element - wait for W to get back up again - e - q - w to reposition to push them into wall - ult - q
You can kill most squishy midlaners this way.
In teamfights force fights in the river or jungle. If you fight in the river you don't even need to hit your ult onto a wall you can just straight up ult them and they'll get stunned. If you're not in a tight space or the river, use your bush q to sneak and get in a position to ult someone into a wall.
Her build is normally:
(if they're building armor) or
(if they have healing)
or 