Notes you must keep track of:
1) On the start of the game, you have an option, Either you buy 3 potions or refillable, I would recommend to the more passive zed players to go refillable as it's way more cost efficient than 3 red pots, However - If you are looking to trade/fight alot in the lane you are in, for example - A Fizz lane, the fizz will 90% of the time look to Q + W you, and in that lane you would need the extra red potion, but for an example a lane like lissandra you can go for refillable, as she can't do much to you besides small poke.

2) On your recall you should always look for having serrated dirk as your first purchase, it is a huge itemspike and is very necessary for the laning phase, as it will be most likely the source of your next kill, zed spikes really hard with that item in the beginning if you manage to not fall behind before lvl 3~5, if you do manage to fall behind, pick up a vamp scepter/refillable potion if you can, sustain for surviving the laning phase so you wont fall even more behind, i'd suggest always going for dirk tho.
If you somehow recall with less than 550g (cost of vamp scepter after you bought long sword), pick up another long sword and if possible buy a control ward also.
If you are feeling a bit lost in lane, and taking too much damage, can always pick up cloth armor / null magic, that you can always complete them to boots anyways later on in the game.

3) Let's talk about the core build, your mythic item is going to be eclipse, as it is in my eyes the best mythic of the lethality items to buy on zed, it provides you stuff that zed uses in all aspects, movement speed, omni vamp (healing), and damage and on top of that a shield. This is in my eyes the best lethality item for the stats it gives, prowlers claw can be used, but i would really recommend not picking prowlers claw over eclipse.
Second item is yommus ghostblade, I'd recommend this item because when you pick up eclipse its going to be around min 10ish, and teamfights/mini teamfights are gonna occur very soon around drakes, ect. Yommus is one of the items that allow you to go in a fight, It allows you to flank easily and catch up to enemies, If the situation allows it, build yommus. (we'll talk about which situations don't allow it)
Boots option - this is pretty simple, enemy has a ton of cc, pick up mercs (even if they dont have AP champs, if they have alot of cc you really should pick up mercs, unless they have like 2-3 marksmans/onhit champs like yasuo or yone, then you should pick up plated steelcaps).
If enemy has a ton of AD damage and not much CC - plated steelcaps.
If enemy has a mixture of AD and AP damage, not alot of CC - recommend to go lucidity boots.
Now for the final item we'll talk about in the core build is ravenous hydra - this is a core item allthough its at the end of the list, reason being is that for ur 3rd item you should buy either ravenous hydra, or one of the situational items listed below, we'll talk about that right now.

4) Situational items - These are pretty simple:
You buy them based on the current situation of the game (Snowballing, enemy has alot of armor, ect)

Black Cleaver - If you see enemy has some armor, and your team composition has atleast 2-3 AD champs, you should buy black cleaver. IMPORTANT: Always let your teammates know when you are buying black cleaver, as some top champs would like to buy black cleaver aswell, and buying 2 black cleavers would be a waste as its mainly bought for its passive (The passive doesn't stack if someone else has black cleaver aswell)

Serylda's Grudge -If you see enemy has some armor, but your team comp is mainly AP, while you are the only AD, invest in a last whisper, and complete it to Serylda's Grudge - This item is mainly used for the armor pen%, even tho it gives some other good stats aswell.

Edge of Night - If enemy team has champs like: Blitzcrank, Thresh, Lissandra, Skarner - You get the point, you get hit by their main ability and you die (blitz q, thresh q, liss r, skarner r), You would want to buy Edge of night, because in most cases you go in fights and they will INSTANTLY use their CC ability on you, edge of night can prevent that from happening. IMPORTANT: give up edge of night 3rd if they have armor, go for the first two options listed above.

Sterak's Gage - I personally like this item as a last item if you have the slot for it, it's a very good item mainly used for survivability after you engage in teamfights, mostly leaving you one HP, but you still live, It's just a good item overall, recommended as situational because you should never buy it 5th or 4th item, always 6th item.

Umbral Glaive - Very situational, If enemy team has stuff like shaco, teemo, or a bunch of assassins, this item is recommended, for teemo and shaco - to clear shrooms and boxes, obviously. if they have a bunch of assassins - this item is very important to clear out vision so the enemy team assassins wont be hiding in bushes while having vision on other bushes, if you are versus assassins, the game will be a vision fight game.

If any of these situational items, don't fit as an item after eclipse and yommu's ghostblade, buy ravenous hydra.

Zed Guide
About me:
Been playing zed around since his release, mainly started one tricking him since season 8.
I've managed to hit masters on the EUW server with 80% winrate - SoloQ without a duo.
I've managed to hit grandmaster on the EUNE server with around 75-80% winrate aswell solo.
I have never really thought about making a guide, but recently with S11 changes, new items, I figured many people are confused, looking for answers on what to build, and how, and I figured I could use my knowledge to help those who wish to know what to build/when to build what.
If you are interested in watching my gameplay, or watching me play, you can find me streaming on twitch every now and then, I will do my best to get daily streams as it is a though time at the moment in school beacuse of final exams.

My Twitch Channel: https://www.twitch.tv/swighter

There is also a montage of me on youtube, if you are interested.
https://www.youtube.com/watch?v=iKARArKFvLo&t=1s&ab_channel=RayLOLs

My Youtube channel with some montages I did myself:
https://www.youtube.com/channel/UCEgt4fN32V0oy_nba4tfNdg