Hello First of All i Want to Apologize For My Horrible English xD
sry About That Anyway I Have Been Playing Zed For Almost A Year Now i Want to Share My knowledge of This Champion so Let't Jump Right in to it..

3 Greater Glyph of Scaling Cooldown Reduction
This Gives Your Nice 5 CDR For To Cap it At 30 With Your Masteries And Your Brutalizer
u Don't Need More Than That Cuz You Are An Energy User
6 Greater Glyph of Magic Resist
Some MR For The Enemy Mid Laner
9 Greater Seal of Scaling Heath
u Can Replace These With Armor if u Are Against an AD Mid Laner
9 Greater Mark of Armor Penetration
This is Better Then Pure AD Cuz We Are in The Tank Meta Right Now Everybody Have Tons of Armor
2 Greater quintessence of Attack Damage
Damage Can Be Replaced With Armor Pen if u Want
1 Greater quintessence of Life Steal
i like This For Some Early Sustain in The Lane Also Can Be Replaced

i Stole These From Faker So They Are Probably Good

Max Q Then E then W And R when You Can ofc
u Can Max E First if u Are Against a Melee Champion it Might Work But i Don't Recommend it

1- Youmuu's Ghost Blade

in Most Cases This Item Will Be Your Second Item Perfect Item on Zed Gives u 30 AD which is Always Needed on Zed 15 Critical Chance 10 CDR
And a Nice Active To Help u Take Down Towers And Even Buffs in Early Stages of The Game And Some Movement Speed to Help u Escape or Catch Someone i Think This Item is a Must on Zed

2- Blade of The Ruined King ( BORTK )

Many People Think That This Item is Not Good on Zed Anymore But i Think That This Items is Simply Amazing on Zed it Gives u 25 AD Which is Nice And 40% Attack Speed This Good Because Zed Base Attack Speed is Low And it Help u Wave Clear Better
And Some Life Steal Which is Always Needed
And it Gives u an Awesome Passive Which Have a Synergy With Your Passive And it Enables u To Fight Tanks And Make Your Auto Attacks Deal So Much Damage
And Finally The Amazing Active That Helps u Stick To Your Target or Run Away From Them Over All i Consider This Items to Be a Must on Zed And This Will be Your First Complete Item

3- Last Whisper
Not Much To Say Here This is Must Must Must on Zed Not Replaceable At All

4- Hydra

ok if You Are Comfortable on playing Zed With 3 Active Items Get This it's Really Good on Zed But u Can Replace it With Black Cleaver or Blood Thirster or Infinity Edge or
Maw of Malmortius

For Boots u Always Gonna Be Building Mercury Treads But if The Enemy Team Have no CC or Magic Damage u Can Go With Ninja Tabi And Mobility Boots if u Are Really Fed And u Roam a lot
And Your Defensive Item Most of The Time Will Be GA But u Can Go Banshee's Veil if Needed

ok so u Are an Assassin so u Need to Focus of The highest priority Target And Kill Him
Find Someone That is Not Tanky And Does Not Have Zhonya's or QSS
if There is None Get Black Cleaver Instead of Hydra For The Armor Pen And Go After The Tanks
or u Can Just Split Push

Pros
1- High Damage
2- Amazing Wave Clear
3- Really Fun To Play
4- Very Stylish
5- Strong All Game
6- Easy Last Hitting With Your Passive

Cons
1- Easily Countered By QSS-Zhonya's
2- Squishy
3- Energy Hungry
4- Banned 90% of The Time in Solo Q

Combo Number 1 - Easy Target

So let's Assume That u Have Blade of The Ruined King And Youmuu's Ghost Blade And Hydra
so First of All u Need to Bait an Ability From The Enemy And Dodge it With Your Ult That is not necessary ofc if Your Opponent is Running And You Can't ULT Him Use Ghost Blade To Get Close To Him Then ULT Then Activate Your Hydra and possibly Ignite Then E Then Activate Your Blade of The Ruined King To Slow Him Makes it Easier To Hit Your Q And To Stick To him Then Place a Shadow Behind Him And Go to it so u Can Dodge Another Spell Then Q That Way u Can Hit 3 Q's And Maybe Auto Attack Him as Many Auto Attacks as u Can Then Go Back To Your ULT Shadow

Combo Number 2 - The Unseen Shadow

so if want to Kill Your Enemy as Fast as u Can Because u Are Low On Health Or He is Fed And Can Kill u Fast Use This Combo
First Use Ghost Blade And Throw You Shadow on The Side of The Battlefield And ULT Him Ignite
Hydra E Then Go to Your W Shadow Q on Him Then Go to Your UlT Shadow Auto Then Flash Away
That Way u Give Him\Her no Chance To react

Combo Number 3 - i Don't Know What To Call This But

u Use This Trick To Get Stick To a Target Or Get Close To Another One
For Example u Are Chasing Nidalee And She is Moving Near The Wall To Jump Over it With Her W
u Can ULT Her When She is About to Jump so u Can Travel With Her Across The Wall u Use This To Stick To Her\He or To Get Closer To Another Target This Can Work With Flash if u Time it Correctly

Combo Number 4 - Defensive ULT

Use Your ULT To Dodge Some Spells Or if u Get Ganked And Your W is on CD u Can ULT The Jungler And Then Away To Drive Them Away From Your R Shadow Then Go to Your R And Get To Safety

1- Use Q To Last Hit And Try To Position it so it Can Kill The Minion As Well The Enemy Laner
2- Try To Hit Your Enemy Laner With Your Q Directly Without Going Throw A Minion To Deal it Maximum Damage
3- Use W Wisely Because if u Got Ganked While Your W is on CD u Will Be Forced To Use Flash or Die in Most Cases
4- W+E To Slow The Enemy so u Can Q Him Easily
5- if He is Hugging His Turret u Can W+Q for Some Damage But This Might Kill u if The Jungler Ganks Because Your W will Be on CD So Use This Combo Wisely
6- u Can Do The W+E+Q Combo Then Jump To Your Shadow to ULT Him And Kill Him or For an Extra Auto Attack To Maximize Your Harass
7- Don't Be Afraid To Burn Flash To Get Close So u Can ULT Instead of Using W To Maximize Your Damage
8- u Can Use Your W To Provide Some Vision Over Walls or To Check Bushes
9- Your W CD Get Reduced if You Hit An Enemy Champion With Your E Keep That in Mind
10- u Can Use Your E Twice With The Same Shadow So u Can W+E+Q Then E Again if He is In The Range of Your W ofc
11- u Can Use W+E For An AOE Slow

This is My First Guide I'm Very Positive That it's Gonna Be Horrible But I Hope u Leaned Something New ....