Hi, I'm Triipnotic, I haven't been here long but I have spent months on end playing this game 18 hours a day. I started this year at level one and finished this year at Silver 3. This is my first guide ever, so it'll be reworked over and over again through out Season 6. Give me all of the feedback you can to help make this guide the best it can be.

FOR AP

So pretty much what I do here is I start with your standard long sword and 3x health potion. On first back I like to try and get merc treads and at least another long sword, because pretty much any item that you build into will require two. Depending on if you are getting "rekt" or doing pwnage, you can get a hexdrinker to give you that very helpful shield and magic resist.
I would then go for his damage items. Because Zed builds a lot of lifesteal, I would go with the defensive item Death's Dance, providing a useful 10% cdr, plus a hefty amount of healing from damage output. Then build your Botrk or Bloodthirster depending on how well you are doing and your gold income, if you can get the B.F. Sword, get it. If not, go for the attack speed and lifesteal the Botrk has to offer.

After those items are said and done, you can build your hexdrinker into a Maw if you haven't already, or switch it out for something more powerful to your liking.

Situational: if you feel like you have to chase a lot, get Youmuu's for more cdr. If you have to deal with a tank (Nasus) get a Black Cleaver, adds some health to you, cdr, and melts armor away.

FOR AD

This how I would do my build vs and AD, skipping all of the MR items for more damage out put earlier. Ninja Tabi provide for some good armor, and if APs are getting annoying get yourself a Maw whenever necessary. But you are building pretty much to just do a whole lot of damage. Situational: Also a Guardian Angel will give you the Armor and MR, necessary to defend from both the ADC and APC of the enemy team, plus you have a revive from it, that will put you back in the fight, hopefully with the cool downs necessary to do damage and get out of there.

Okay,

so this is sorta different from the other masteries you are seeing.
Pretty much everyone is doing it different, and there is no right or wrong way to do it either.

What I prefer is the AD route.

What is notable is I get Natural Talent, to scale my AD along with a little AP to buff Zed's passive.

Deathfire Touch will also get buffed by Natural Talent, and provides a little security when you go in for a kill.

I prefer this over Fervor because Fervor only works for your basic attacks.

While you are using Zed you will be doing a lot of jumping around and throwing Q's, auto attacking in between. Most of your damage comes from landing your abilities.

Now there are two ways I would use my last 12, the first is what I have put up there.
Why? Merciless is great for an assassin.

If you were to put all 12 into one of them, I would put it into Cunning, getting Merciless and Bandit for extra damage and gold.

If you were to do something different, I would split 6 and 6 into each, getting Savagery + Secret Stash and Unyielding + Tough Skin. What this does is add extra resistance, plus extra damage and sustain.

I always start with Q, and then get my W, then have my E. W+Q is great for poking the enemy and with your E you can slow him so your Q is less likely to miss, plus you can activate your W to jump on the target and get a couple auto attacks in before going back to farming. Rinse and Repeat this until level 6 and you should be able to all in him/her once you have your Ult up.