This is my first guide and I decided to do it on both my favorite champion and the one I've had a lot of success on, even after all the nerfs, Yasuo. Yasuo is a very high skill cap based Assassin/Bruiser. He's counterpicked fairly easily, and is in general a risk-reward champion. However, if picked into a favorable match-up, and you know what you're doing with him. You'll have a great time :D.

Runes:
I run 10 percent crit runes On yasuo simply because when combined with Way of the Wanderer's synergy with Statikk Shiv and Infinity Edge, it gives you a 100 percent chance of landing a critical strike. This is huge, as it boosts your late game scaling considerably. The scaling health seals and mr glyphs are when facing an AP champion in the mid lane, and they also allow for both a fairly stronger laning phase and a stronger late game.
Masteries:
Mostly standard AD Mid Laner runes, except no CDR. Not needed on Yasuo, put the points in other places.

I recommend to both start with and max sweeping blade first. Maxing Steel Tempest will make you have a higher damage output, but the sheer utility and outplay potential you gain from Sweeping Blade far outweighs that damage.
Wind Wall barely has any benefit from being maxed. Don't bother until level 14.

Pros:
Strong farming
Scales exceptionally well.
100% crit chance.
Great duelist.
Good split pusher.
Good team fighter.
Cons:
Extremely high skill cap.
Hard to play properly.
Most people will tell you you're building and upgrading the wrong things. Ignore them. They're wrong.

Standard last hitting. Except all your abilities are on a low cooldown and have no cost related to them. Use both Steel Tempest and Sweeping Blade to get minions you miss with regular last hits.

Start with a Doran's for early sustain. Or Boots if faced against skill shot oriented champions such as Ziggs and Xerath.
Rush a Statikk Shiv for the early spike in power, plus the crit chance and attack speed both scale exponentially on Yasuo. And the movement speed is also very nice as it lets you proc your passive faster.
Then go for Berserker's Greaves if even or ahead. If behind, go for your Defensive boots, either Mercury's Treads or Ninja Tabi.
Then Infinity Edge. Major Power spike and finishes off your 100% Crit chance.
Next go for a life steal item. Either:
Blade of the Ruined King
Ravenous Hydra
or Bloodthirster.
BotRK gives you greater 1v1 potential and tank shredding prowess, so usually i recommend that item over the other two. However, in certain situations the others could work as well.
Skip Last Whisper, you don't need it. Last Breath's Armor Pen is enough.
Then end with two tanky items. Take your pick, they all work besides Spirit Visage and Frozen Heart, as stated before, CDR is useless.