ESSENTIAL TO YOUR BUILD!
The build listed is specifically designed to have no waste in gold and to go back to buy ONLY at specific periods.
Buying a long sword first and nothing else while in bot lane gives you enough time to farm and wait for gold gain to get a Vampiric Scepter. THIS IS IMPORTANT! NO NO NO DORAN'S BLADE! This Scepter will sustain you throughout your lane fighting! Go back to get it when you get decently low on health.
Now, DO NOT BUY ANYTHING ELSE UNTIL YOU BUY YOUR B.F. SWORD! Not shoes, not a dagger, NOTHING!
The combo of Scepter and B.F. Sword will win you lane by massive amount of poke ability and sustaining lifesteal.
DO NOT BUY A BLOODTHIRSTER OUT THE GATE! You need shoes next for mobility and attack speed and Greaves cost as much as a Bloodthirster.
Next build into Runaan's Hurricane. THIS WILL WIN YOU TEAMFIGHTS!!!! When you shoot off rockets, the Runaan's will copy the explosion on impact effect which will layer onto other nearby champions and start giving you massive AoE lifesteal and damage to the enemy team.
Next build item is to get an Infinity Edge for that extra damage you need when getting critical hits.
THEN, AND ONLY THEN, buy a Bloodthirster and another Vampiric Scepter to insure you stay alive under some bombardment.
Then to finish it up, get a Phantom Dancer for extra critical chance and so you won't be hindered during running away by all those minions.
If you want to you can either buy another Bloodthirster or sell the Vamp Scepter to get something that involves getting you necessary defense such as a Negatron's cloak.
Runes - Durability and Sustain
Your main goal with runes is to get Jinx durability and health sustain in lane, the less she has to go back for health means more farm. If you follow the item build guide up to the first B.F. Sword you should be sustained in lane for a long while.
Build your sustain and life gain accordingly in your runes
Spells - Running Away
Usually flash and ghost for getaways is a good choice, you can also substitute for flames if you need to get some last second damage in for a kill to get away-which due to her passive- is pretty much like a ghost every time you get a kill, assist, or building destruction.
Farming - Runaan's = Gold for days
Farming is basically just using the rocket launcher to kill the weaker minions and the minigun for lifesteal off the stronger ones.
HOWEVER! Once you build into Runaan's, you will get massive AoE lifesteal+AD farming ability because it copies the explosion onto two other enemies.
This also gets you hits on pretty much every champion in teamfights, so whenever an enemy dies, you get either a kill or assist and her passive triggers to chase after more enemies or just run away.
Using a full build, I have farmed 96 minions in roughly one minute (it was in an onslaught at base, but still, that's cray). I've also scored a quadra kill because the enemy team bush clumped and I just hit them for 4-6 attacks and an ult. They were just DONE!