Zed is a Assassin Type which requires a high amount of skill cap
Zed is Weak in the early game bit makes up for the late game when well fed

I go for
Quints of Attack Speed or Attack Damage
9 Marks of Lethality or Attack Damage
9 Seals of Armor or Health
9 Glyphs of Cooldown Reduction or Magic Resist

Notes...
Attack Speed Quints are great for the whole game so you can have an easier time with minion spree.
Attack Damage however Would be great too but wont really affect Zed that much other than a slight boost in damage
Marks of Lethality is great in the early game to deal damage since lethality is basically armor penetration.
Marks of Attack Damage will make up for using Attack Speed Quints.
Seals of Armor or Health will make you tankier to have a slightly higher chance of sustain in the early game to avoid feeding.
Glyphs of Cooldown can help you take down an enemy quicker but will rely on you abilities such as E or Q skill.
Glyphs of Magic Resist is great to compensate against any opposing lane that focuses on AP or Magic like Veigar and Teemo

As for Masteries I go and use Thunderlord's Decree as my Keystone
I go for 12/18/0 in Masteries (IM VERY SORRY but I dont know how to have picture and such on this guide and this is my first guide i did this on mobile BTW) dont pick anything that has to revolve around mana on masteries

As for Items
Early Game:
Doran's Blade and Potions
rush Hexdrinker if you are against any AP Based champions but dont rush Maw of Malmortius after that

Mid Game:
(this is written in order of how I build it)
Youmuu's Ghostblade,Boots(Choices below),Duskblade of Drakkthar
For boots I go for Ninja Tabi VS Tanks,ADC, or Melee champions.
Mercury Treads VS AP or Mages
Ionian Boots of Lucidity if You are well fed

Late Game:(this will contain the full build and options)
Youmuu's Ghostblade,Ionian Boots of Lucidity,Duskblade of Draktharr,The Bloodthirster,The Black Cleaver and Death's Dance

Options:
You can replace and of the items excluding the first 2 items excluding the boots
Maw of Malmortius
Statikk Shiv (IDK WHY I would use this other than for the Attack speed boost)
Blade of The Ruined King
Edge of Night

Always go for Razor Shuriken (Q) as the first skill
Either go for Shadow Slash (E) if the enemy is non ranged or Living Shadow (W) if the enemy is ranged and to play safe
this is the sequence for the first 6 levels NOT COMBO

Against Melee:
Q,E,W,Q,E,R

Against Ranged:
Q,W,E,Q,E,R

I max Razor Shuriken first followed by Shadow Slash and the last Living Shadow,Take Death Mark whenever possible :)

Your Main combo when you dont have your Death Mark is
W-E-Q-E only use W when the enemy is close to death and to close or to make your next Q skill count...
you can also use this against any enemies near death even at their tower but make sure there arent any enemy supports for the recalling enemy

R-E-W(place where the enemy can escape)-Q-E-R only reactivate W to actually kill the enemy if they survived its best to use this against squishies or any enemy that is wrecked to gain the extra damage from killing your enemies

I hope this helped you but I dont know how to actually make a proper guide on MOBAFire since Im on mobile but anyways thank you guys for reading this guide I hope it will help you to be a better Zed user
I play on Garen PH servers BTW my IGN there is ReverseNeo my top 5 highest mastery champs are Zed,Master Yi,Warwick,Yasuo, and Lee Sin
once I learn how to make a proper guide with pictures etc. I will remake this but keep this so I can remember my first guide...
anyways Good Luck :)