This is my First Guide. Zed is an Easy champ with great abilities.
This is a Guide for the toplane.
The Items I listed are usefull against a "normal" Team but if you are against a very hard Ap Enemy it is may usefull to buy Maw of Malmortius or against a heavy tank Team the Last Whisper although you already have the Black Cleaver.

Easy to farm with Zed because of his Passive.
It is also easy to farm against Ranges enemys because of his W,E,Q-Combo without getting poked.
With Living Shadow you can come near to the enemy so you can use Shadow Slash to cause damage and at last the Razor Shuriken to hit him if he is already out of the ranged of Shadow Slash. This Combo also works for minions because through this you get enough farm.
The only Contra Point with this "Combo" is that you have low Energy after that and it needs to restore a Bit.
In General it is easy to farm with Zed if You Knowledge to Play Jim a Bit.

Pros:

+no mana
+Easy to Farm with the passive
+His Ult does Great damage
+Easy to escape with Living Shadow
+You have a slow with Shadow Slash

Cons:

-his Energy needs to reload
- Easy to farm so you could get ganked often if you push too much

Through the Greater Mark of Attack Damage you have enough damage to cause good damage and farm well at thebeginning.
The Greater Seal of Armor will give you enough Amor to survive against an AD toplaner who casues hard damage.
The Greater Glyph of Magic Resist gives you enough MR at the beginning to hold the lane against an heavy AP opponent. The Runes are good because through them you should have enough Amor/MR to survie and enough Damage to make kills right at the beginning.

Flash is your Core Spell because it gives you the Chance to escape when you pushed your lane too much so you get ganked.
Ignite is the ultimate Addition to your Ultimate Attack. Also it is usefull because than you do not have to dive the tower if you are able to ignite your enemy when he is low life.
Exhaust is Not recommend because Your E already slows. And Zed is already mobil.
Heal is not necessary because of the lifesteal you got in endgame.

Doran's Blade and Health Potion give you sustain and is therefore a Safe Start. I prefere the Start with Long Sword and Health Potion because you can Rush The Brutalizer earlier.
The Items give you enough Damage or amorpenetration to cause much damage against Most enemys.
Ionian Boots of Lucidity give you the cooldown reduction Therefore you can use your Abilities more often.
Maybe you have to buy some MR at the beginning which I Would do in form of Maw of Malmortius but only if you are against an AP toplaner WHO Deals too much damage against you. I normally use Bloodthirster for Zed but you can also decide between Ravenous Hydra or Blade of the Ruined King but Blade of the Ruined King would give you less damage. It is not the Big difference between using Ravenous Hydra or Bloodthirster because both of them give you enought Lifesteal and Damage so you can feel free to take which you prefere.
Randuin's Omen and Frozen Mallet will make you a bit more tanky so you can survie a bit longer while Frozen Mallet gives you more damage aswell.

The 21/09 Masteries Are Perfect for Zed.
The cooldown Reduction is also very usefull because through this you can use your abilities More often, what means that your Enemys have less Time to get on full HP again.
Through Double-Edged Sword , Sorcery, Butcher , Brute Force , Martial Mastery , Warlord and Havoc you get enough Damage to get kills in early Game. Through Devastating Strikes you'll get some amorpenetration which is also good if you have to play against a Tank in Early Game. Through the Defensiv masteries you get the amor,MR and health you need to survive in lane longer.

Early Game

It is very important to get enough farm for the Items you want to consum.
Doran's Blade and 1x Health Potion will give you sustain in the lane to Stay longer.
But Long Sword and 3x Health Potion will give you a bit more Damage and you'll get the sustain with the Health Potion
It is very important to get many Kills in early Game. So you can give your teammates some Kills through your damage.

Mid Game

Your duty now is pushing the Lanes to get some towers.
Your task in Teamfights is easy: just do much damage so you can give some Kills to your mates if you want to help them.
You are Not the One WHO engages. Because you are easy to kill like an ADC.

End Game

Your task and Aim should be to get the inhibiotrs down what should be very easy because you have so much damage.
If you have finished all your Items you are able to get an ACE with your Team and finish.
The defensiv Items give you the Durability to Stay Alive until you and your team Killed all the oponents.

This is not a normal assasin Guide but I Hope You like it.
I like Zed because he is very easy to play and his Farming Combo is very easy so everyone should get a high farm with him.
He is very mobil because of Living Shadow and has also a Slow with Shadow Slash
He gains much damage through the items I listed and is also a bit tanky which is nice so you can life longer if you are in team fights. I like this champion very much and I hope you will,too.
I hope my guide is a bit helpful for you.