Starting off lvl 1
When you start farming at level 1 in mid lane be some what passive until possibly 3 or 4 unless you are getting a gank this build relies hard on farm so ... farm farm farm away
Lvl 3 - 4
by now you should at least rank 1 boots and brutalizer and possibly a vamp scepter if you had gotten a kill. But by now u should be able to poke with your q efficently so while there are no minions in lane you should be doing this.
This is where it become very important that you start getting your lane in control Well your Zed enough said. Dont make any jumps until you know that you can get a kill and be sure not to over extend yourself in laning phase. But if you are the roaming type now is a better time than never to do it.
lvl 7-10 and beyond
by now you should have a couple kills and hopefully no deaths but things happen you should have up to bilgewater cutlass for your blade of the rune king if your lane happens to go down then roam around and farm until you see an advantage such as weak or 1 v 1s that you know you can win because if your dead you cant farm. By the time you get your 2nd BT you are the Carries Worst Nightmare
Purpose in team Fights
KILL THE ATTACK DAMAGE CARRY possibly the most important job that you can do. Most of the time in team fights the carry is the most fed and with them out of the way it becomes a easy 4 v 5 fight Even if the ADC has GA or Guardian Angel use your ult on the AD carry don't consider it worthless because the AD carry is down for a couple seconds for the Res in the mean time that E of yours does damage when the ad carry gets back up jump in his or her face and kill her again. after that your job is clean up and don't die.