Hei im BossDos. Im new at Mobafire and this is my first real guide.
This Guide isnt a way to win a game, that is your work, not mine.
Ill just say how i build this champion and why i do this, if you do this or not its not my Problem.
Dont flame when you lose with this Build, u have to do some work, too.

I like this Rune-Set more then others.

AD Runes [Flat] & Quints
You will do enought damage without great damage items and you can set the priority to Items that will help you more.
Like Statiks. You dont need Armorpen-Runes, because of your ultimate.
U can get 5 Crit Strike Marks in here, to have 100% critstrike late and 10% early without items.

Health Runes [Scaling]
You dont need Armor in 90% of all matchups, cuz u are on midlane and your enemy will be a caster normaly.
So the Health Runes are quite strong on you, u are scaling very good into mid and lategame.
Thats where u get your power spikes and u are an Assassin, when u engage with your ulti into a whole team u will life a lot longer with 200+ life.

Magic Resistance Runes [Flat (or Scaling)]
I like the flat runes more, because normaly u are playing an the mid lane and u are standing against a caster. That will help u against the caster and u can hold the lane longer and farm more.
If u arent on Midlane select Scaling Runes for the Late Game Advantage.

You are an AD Carry on the Midlane thats because we select and offensive Mastery with 21 / 9.
But you can invest 6 Points more in the Tank Tree to Hardiness and Resistance for more substain, but then you will lack a little damage.

You dont need Sorcery because your Q gets CDR from Attackspeed and the other CDs, like your E, are so low that u dont need CDR for this.
Double-Edged Sword isnt so good at Yasuo because you arent tanky, and you need your life for Champion Engage. Because of this we will take Butcher that will help us to take farm.

You normaly start on Doran's Blade, u get HP, AD and Lifesteal, this will help u stay longer in the lane cuz the lifesteal will heal some harass and minion dmg.
You can start on Boots it will help u charge your passiv much faster and u can buy 4 Health Pots. Only would recommend it when u are get countered in Lane from something like Ziggs, Xerath or a Fizz.

In Early Game i would build Avarice Blade, u get 10% Crits, with your passiv 20%, quite a nice DMG spike early, and u get Gold advantage, cuz every lasthit gives u +2 gold and every 10 seconds you get 3 gold.
After this Item i qould buy a Pink and 2 Green Wards and perhaps my T1 Boots, then going for Zeal, cuz u want your Statikk Shiv.

The first great item you get is Statikk Shiv its awesome on Yasuo. U are charging the Passiv very fast with all your speed u get a Attack and Movingspeed boost und u are having 40% Crits.
Now we can build Blade of the Ruined King, you got 40% attackspeed, 25 Ad, and 15% lifesteal, that gives you more substain and the active is realy nice.
The Next Items are Infinity Edge, now u have 90% Crit, 9 of 10 hits are critting and this with 250% Damage, hello? You are scaring as hell for everyone except for the Tank, but nobody cares about the Tank.

Now u are buildung Guardian Angel for the Substain of 50 Armor and 40 MR, that will not make u tanky, but helps u to get the whole Enemy Team down, before your team is getting killed.
The Last Item is depending on the Situation, if the enemy Team is a little tankier, build a Last Whisper.
If they are AP Heavy a Spirit Visage, Banshees Veil or a Mercurial Scimitar will do it also.

You can Change Statikk Shiv for Phantom Dancer a lategame, if you need it. Then u get 100% Crit, but you will lose the passiv from Statikk Shiv what is realy nice.

Why this Sequence? You need Sweeping Blade to engage and it has a very low CD on low Lvl you cen farm with it very easy, chase your enemy and can escape when you left a enemy minion in your back.

After this you need Steel Tempest because it is your Main Damage Scource and it ceals incredible Damage on low Lvl and you can make it stronger with Attackspeed cause it will lower the CD of your Q.

Then you need Wind Wall, now you can defend yourself against Enemy ranged Attacks, and the passiv helps you to charge your passiv.

The First Skill you Max is your Q, because its your best skill and then your doing your most damage. After this you max your e, cause of extra damage and such a low cd that you can instant use your e.
After this you max Last Breath, cause the Armor pen is awesome against all enemys and you can easy kill enemys when they are runing away. Send Whirlwind after them, press r and have a nice day.
The last Thing you master is your W, because it does his job at lvl 1 as on lvl 5, it only becomes bigger and stays a little longer, and of cause it reduces the cd.
But it will do his job at lvl 1 as well on lvl 5.

Pros [+] Snowballs strong [+] Strong, perhaps the strongest, Lategame [+] High Damage Output [+] High Mobility [+] A great potential to get Multikills with ult [+] Great Execution capability with the right Teamcomp and his ult.

Cons [-] Weak Early [-] strong fragility against cc [-] Without Latgame Items very squishy

Yasuo is an champ who need Farm, for gold and exp, because he is very depending on items and lvl, cause of his weak early game.

You have two Abilitys for farming your E, with his low CD of a half second, you can very nice lasthit with this ability and powering it up for engage to the enemy champ.

And your Q, it does nice damage without items and you can lasthit minions at 40-50% of their max. Life, you can also stack it for the engage on enemy champ.

So enemy minions are your best friends, they will make you strong.
Farm them and dont engage the enemy champ till you got lvl 6 and your full early items.
If the enemy champ builds pressure on you, get your jungler to help you, and shield you with your w, as long you can to farm in save position.

But in all chases, dont fight in Enemy in your early, before getting an Avarice Blade.
This Item is your first DMG spike, but we dont build it your duelling. We need it to get Goldadvantage, when we are farming and this is the only thing we will do.
Sometimes you can harrass your enemy with your Whirlwind and your e, but never go all-in, without jungle assistance or before u got an Statikk Shiv.
You will get your Statikk Shiv fast, cuz we started on Avaric and normaly we should have the goldadvantage on lane.

We will wait for our Lvl 6, cuz then we can fight, but just use your ult when u can 100% Kill your enemy with your ult, or your next AA / Q / E.
Its a Great Tool for kill your low HP Enemy behind his tower, cuz u are invulnerable while ult, the tower will not do so much DMG to us.
And when we secured the kill we have Tower Focus, so we should move out of Range with moderate Speed.
Cuz of the Last Point, we WILL NO dive with our ult, if the tower will 1 or 2 hit us.
1 for 1 isnt worth, so dont dive with ult when u are low HP.
You can only do it, when your Summoner Heal is Up, cuz it can save your life, while securing the kill.

Yasuo is a Champ u can pick against many Enemys, even against his hardcounters.
But then u will need the right Summoners on him.

One Summoner we always choose, will be Flash, you dont have a real escape spell, cuz your e needs an enemy minion to escape well.
So we pick up Flash, it can secure kills, save our life and its just awesome.

The Secondary Summoner is adaptable, the most people taking Ignite.
Its quite strong, does true dmg as a DOT, and can secure kills and will lower Heal/Lifesteal of the enemy.
But in 60% of the chases i wont choose it, cu we dont need it. We have enought DMG to finish the most enemys.
I would only choose it, if my enemy Team has something with hard Heals [Gragas/Mundo/WW).

My favorite Summoner is Heal u get so much pressure from it, cuz u dont have Mana, so u dont need to B, cuz of OOM and u dont need Blues.
U can Stay in Lane forever, with your Dorans u got additional lifesteal, so u dont need to B often.
And it can secure u Kills on Low HP, when u dive with your ult, u take a towershot heal yourself and ran out.
Also u can take it if your enemy has high Poke, to all-in on 100%.

An Third Place there is an underrated Summoner Cleanse, it will drop all CC and DoTs from u.
Quite strong and u dont need to Build Quicksilver Sash, its rly expensive.
Against Hard CC Champs like Veigar, or Syndra its very usefull. Also a Fizz will cry, when u Cleanse his ult and his Combo is broken.
Then u can simple kill him.

I hope this guide will you, to master this champ and win games with it.
Its not a Key to the win, but it should help, when you have the skills and the mind.

Im very sorry for my bad english skills, if their are great mistakes, you can say it i will eliminate them