Yasuo is a high skillcap AD assassin, usually played in the midlane. Before reading this guide, I'd just like to say, DO NOT PICK YASUO UNLESS YOU KNOW YOUR TEAM WILL HAVE KNOCKUPS DON'T EVEN THINK ABOUT IT. Alright, with that out of the way, let's talk about why you should pick Yasuo.

Pros
Great pick potential
Great roaming
Assassin with lots of CC
Easy to farm as
Lot's of mobility for outplay potential and chasing targets
Wind Wall is straight up broken, especially against poke comps

Cons
Bad early game
Need to be really careful with positioning late game
No reliable escape
Bad duelist
Needs coordination with team to be successful

When to Pick Yasuo?
You don't have an assassin or splitpusher already
You have an AP jungler or top laner
You have reliable knockups on your team
Their team has key projectiles to block, for example, Jayce, Ziggs

The reason I have no matchups section is because its relatively straightfoward. Yasuo loses lane to almost all ranged champs due to their constant poke and waveclear. Exceptions would include Karthus because his AA damage is negligible, his resource costs are high, and his Qs are easy to dodge with Sweeping Blade. He also loses lane to almost all melee champs because his W is pretty much useless except against Zed which he actually does quite well against, and most melee champs have better base damage than Yasuo does.

AOE knock-up, near instant, really hard to dodge, plus you'll have a massive tank for the team to deal with after your ult ends, so it adds a bit of safety.

Instant, targeted knock-up, plus the mobility, CC, and damage Lee Sin brings makes it a very good 2-man pick team.

Can chain your knock-ups together, order doesn't really matter, and gives you an amazing health boost to make sure you don't die after using your ult. Also a good AP top laner since you'll need AP on your team.

Not a really reliable knock-up but the shield will increase your damage and she'll be able to peel and scatter the fight for you after your ult.

Same reason as Lee Sin, but trades reliability of a targeted skill for a larger range knock-up and the chance of knocking up more targets.

Huge knock-up radius, great tank with a lot of disruption for saving you after you ult.

Great tank, provides damage reduction after you ult. W + Q combo from him is a guaranteed knock-up (AOE of you're lucky).

Pretty much just an avoidable knock-up, taunt will help peel for you after your ult, and Soaring Slam does a lot of AOE damage while they're CC'd

Huge AOE knock-up, tons of AOE damage while they're stuck in your ult and peels for you after your ult is done.

Bomb delivery service attached to your ult, extra mobility makes sure you're not going to be kited, revives you to continue slaughtering the team if you die while they focus you.

HUGE AOE knock-up with something like a 400$ AD ratio hitting their team while they're getting knocked up.

Command: Shockwave does huge amounts of damage, is relatively easy to hit too, especially when Orianna puts her ball on a champion like Yasuo who'll get piled on the moment he jumps in. Either you'll get a knock-up and Ori will get her ult off after yours, or she'll just find a way to ult them and you get a huge amount of damage off.

Marks
Greater Mark of Attack Speed - It gives you CDR for your Steel Tempest. You can run 9 of these if you don't care about the difference between 90% and 100% crit chance.

Greater Mark of Critical Chance - Helps with getting 100% crit chance. These are optional, take 4 if you want 100% crit chance.

Greater Mark of Attack Damage - You don't need these because its easy enough to farm with a Doran's Blade start, Steel Tempest, and Sweeping Blade, but take them if you're finding yourself missing a lot of farm.

Seals

Greater Seal of Health - Gives protection against AD and AP (the more common one hence why I chose not to run Armor seals) in the early game where you're the most fragile.

Greater Seal of Scaling Health or Greater Seal of Scaling Armor - Run these if you're confident in your ability to survive lane without being harassed, they're a lot better in the long run.

Greater Seal of Critical Chance - One of these plus 5 Greater Mark of Critical Chance gives 5.1% crit chance, doubled to 10.2%, enough to hit 100% crit chance with a Statikk Shiv and Infinity Edge. Completely optional, only if you're like me and really paranoid about the missing 10%.

Glyphs
Greater Glyph of Magic Resist or Greater Glyph of Armor - For defending against the respective damage type. I'd personally prefer MR glyphs just because AP champs are more common in mid than AD champs. Also, don't take Greater Glyph of Armor with Greater Seal of Scaling Armor, diminishing returns exist for resistances (The more you have, the less effective each point is).

Quintessences
Greater Quintessence of Life Steal - You have a weak early game and love to spam autoattacks on your minions (yes Steel Tempest does count for lifesteal). These quints help you sustain from the harass you'll be taking in lane.

Greater Quintessence of Attack Speed - If you're confident in your ability to survive without sustain, go for it. Extra CDR on your Steel Tempest is nice.

NOTE: The runes I run allow me to have 100% crit chance, but with my masteries, running 10% AS worth of runes lets you get the max CDR on Steel Tempest only building a Statikk Shiv

The 21 points into the offensive tree and 9 points into the defensive tree are the best options for Yasuo since he doesn't really really need anything from utility, he has enough mobility and is manaless after all. His main role, assassin, means he should put 21 points into offense to get Havoc , but not all 30 points because he's incredibly squishy, especially in the early game so you'll want 9 points in defense for Juggernaut to help you not die.

For an assassin, it's well worth taking some extra damage to have your already massive damage output amplified.

I chose this over Sorcery because Yasuo doesn't benefit very much from CDR and the bonus attack speed gives Yasuo a lower cooldown on Steel Tempest.

Two points to help you farm really well and the sustain keeps you in lane longer.

Spell Weaving and Blade Weaving are good, but I only had 1 point to spare when I took the essential AD masteries, so I put it into Expose Weakness instead, its just a better one point wonder.

Free AD on an AD champion. Always good.

You're going to be getting crits a lot, so the bonus attack speed will pretty much always be there. The bonus AS will also help you get Steel Tempest up faster.

Armor Pen is great for helping you kill tanks, Frenzy is always bad because it gives next to no AD.

FREE DAMAGE?!?!?!!

Only thing I have to say about the defensive tree is, you're not a jungler and Enchanted Armor is awful, it gives next to nothing and Armor and MR have diminishing returns.

Start Doran's Blade 99% of the time. If you're up against something like Teemo or Quinn and you're sure you're not going to have any chance at winning, then you can start Cloth Armor + 5 potions or Doran's Shield.

Rush a Statikk Shiv always. The passive synergizes too well with your flow and Sweeping Blade helps you get the stacks faster. If you're behind, Avarice Blade is a great comeback item because of all the passive gold you get, especially if you decide to dirty farm the jungle since the small, weak creeps give tons of gold for no effort.

Get one these items. Usually you'll be going for the Blade of the Ruined King because you'll need the sustain for lane and midgame skirmishes, but if you came out ahead in lane, you'll be able to afford a B. F. Sword to build into your Infinity Edge. Otherwise, you'll have to settle for the components of a Bilgewater Cutlass and rush your BotRK instead of an IE. Get the other one afterwards though and that'll be it for your offensive items.

Good alternative for Blade of the Ruined King if you're fed enough to afford the B. F. Sword, and they don't really have anyone stacking health. You'll probably need Berserker's Greaves though for the attack speed to hit max CDR on Steel Tempest unless you have 15% AS in your runes and masteries.

Why no ?

Yasuo is an assassin who's main goal is to use his CC to remove any sort of counterplay the other team has while killing the carries in the process. Compared to other Assassins and melee carries like, Zed, Fizz, Master Yi, Tryndamere, or Fiora, he doesn't have the ratios nor the steroids to fight tanks in a reasonable amount of time or even beat bruisers and tanks like Rammus or Darius. Yasuo sacrifices the damage steroids most assassins and melee carries have for more mobility and CC to help kill the carries, but not so much tanks. Last Breath gives you enough armor pen to duel most champs, a Last Whisper is just wasted because 1v1'ing tanks is a waste of time for Yasuo and could even be difficult depending on the matchup. It's much better to just get a defensive item to help survive in teamfights.

Defensive Items

Only get this item early on (sometime between Avarice Blade and Statikk Shiv) if you're getting destroyed by the other laner, usually someone like Annie, and need to survive their burst.
Great ball of stats, tons of HP and Armor, AS reduction to lower the damage output of ADCs on you too. Really good synergy when cast after you ult since it gives you great self peel after your ult ends.
Just buy it for the MR and to remove one crippling spell from you like Infinite Duress or Scatter the Weak, don't upgrade it into Mercurial Scimitar until you have nothing else to build.
Great for stopping that one spell you can't QSS, like Unstoppable Force, Command: Shockwave or Sigil of Malice. Also gives you a hefty amount of MR and HP.
Generic defensive item for when you're being focused in teamfights but have another strong carry on your team to deal damage while they're focusing you and your respawn animation. Also for when you're getting bursted down by too much for a Banshee's Veil or Quicksilver Sash to save you from.

Boots

Attack speed helps you lower the cooldown of Steel Tempest great choice all around for ADC-type champions.

MR and Tenacity for more survivability against AP champs and teams with heavy CC.

Great choice against AD champs like Riven who'll bully you out of lane if you don't build defensive.

Levelling it up and maxing it first increases the damage and lowers the cooldown, not significantly but its better than nothing. Eventually, this skill will basically become your new autoattack. Spam it in lane on anything you can as much as possible unless you're trying to freeze. The third strike is great poke (if you can land it) for free. Under tower, this skill is amazing for helping you farm so having it off CD faster is definitely a necessity if you're being pushed in, which, most of the time, you will be. The CDR maxes at 55% AS at level 18 though, so anything past that won't lower the cooldown of Steel Tempest.

Levelling it up and maxing it second increases your mobility since it lowers the cast time and the time it takes to jump to the same target again. It also increases the damage by a slight amount. In lane, use this facing the minion's side, that way, you can keep up the bonus damage stacks while not E'ing into the other laner's face and dying for it or away from the wave and missing farm. This skill is also really good for last hitting, but remember to use it facing the minion's side. You can use this in combination with Steel Tempest in order to get an AOE Steel Tempest instead of a linear one. Be aware that this does reduce the range of your knockup tornado. If you think you can get a kill from the knock-up (e.g. when your jungler ganks) try not to EQ until you get the 3rd Steel Tempest to guarantee that you hit your knock-up. If you're being ganked, you can use this skill on a minion or the jungler if he's behind you to get away safely.

One point wonder, increasing the rank increases the wall's width and lowers its cooldown but it'll usually be used to block one key skill so neither of those really matter to a well aimed Wind Wall.

Don't just mash this button randomly. Wait for a large amount of people or their carry to get knocked-up before using it. Otherwise, you'll just get killed during or immediately after you use it because if you don't ult a lot of people, they'll still have their CC, and if you don't ult their carry, they'll still have a lot of damage. Also, try and see when they're about to fall back down because Last Breath resets the knock-up time, so a 1.5s Unstoppable Force could theoretically be reduced to a 1.0s knockup because you were too impatient to count down the time and turn that Unstoppable Force into a 2.5s knockup.

Farm, farm, farm. This is your time to get strong. Don't try and make plays like Dade or PawN. Especially with your nerfed early game, it'll be exceedingly difficult for you to match up against other laners.

Try not to get poked too much, remember, your Q and E have no cost so use them as much as you can to help you farm and dodge skillshots. Your EQ combo is really good for clearing wraiths, so do this once you get an Avarice Blade for tons of easy gold. You can feel free to leave the big wraith up for your jungler too if you don't feel like you can finish them healthily. Your Wind Wall helps you do this jungle farming because you can block the wraith's autoattacks with it since they're projectiles. Your Wind Wall also has no cost so if they have no important ultimate to block, just use it to stop them from getting creeps or harassing you.

Your shield is a great tool to help negate harass, so walk around a lot or just E to last hit not only to keep up the damage stacks, but to build up your shield so you can walk-up and get some extra cs for free once in a while. If you're up against a ranged champ, I'd strongly suggest avoiding the ranged minions unless your melee minions are attacking them. Otherwise, you'll get harassed A LOT trying to walk up to the ranged creeps so far away from yours.

By now, you'll have a Statikk Shiv and they'll have their first item or a Seeker's Armguard and the components for their second. Now, you can 1v1 many of the ranged mages you couldn't before, Statikk Shiv just multiplies your damage by so much, with increased attack speed, more crits, more Steel Tempests, and the passive's burst. It'll still be difficult though to 1v1 other assassins who can avoid your CC or just straight up out damage you like Zed and Fizz if you fail to Wind Wall their skillshots or Riven.

Now, you have two choices. If you co-ordinate well with your jungler and or other lanes with knock-ups, you can begin roaming and picking off some easy kills. Yasuo's mobility, especially with a Statikk Shiv, and CC is really good for these skirmishes. Otherwise, if your lanes are doing well, you can just keep pushing mid, warning your lanes of potential roams from the other mid laner. Being an AD champ, you'll take down towers really fast and after you take down their tower, you'll have the freedom to roam anyways along with increased control of objectives and their jungle.

You'll have to be very careful in your positioning. You don't really have any defensive skills so if you get caught out, you're in a ton of trouble. You'll be hard pressed to actually get 3 procs of Steel Tempest so wait for a teammate with a knock-up to initiate onto a large group of people or their carry or else you're not going to have a fun time waiting to hit Steel Tempest 3 times.

If they have no one on their team like Master Yi, or anyone else who could beat you in a 1v1, it might be worth it to just splitpush and meet up with your team half way while they follow their team trying to stop you from pushing. Yasuo's Steel Tempest gives him great waveclear, and building AD items, he can take down towers very quickly so splitpushing is a very good alternative to just standing mid, staring at the other team, doing nothing. Just make sure their team doesn't have an Amumu or anything that could force a hard engage onto your team while you're gone.

I hope my guide helped you out, please leave any questions or suggestions in the comments and I'll be sure to read them!