Well I may not be a Diamond player but I am a very experienced Yasuo player and I'm in Gold right now.

Average KDA is 3.15:1
9.4/5.4/7.5 (SEASON 5)
I have a 60% win rate with him in ranked.

I think I'm good, I don't lose lane often as Yasuo and my mechanics are on point with him.

This page is for when you're fighting an AD lane opponent or when you think you are.

Marks
5x Crit Chance
4x AD
Seals
9x Armor
Glyphs
4x AD
5x MR
Quints
2x AD
1x Crit Chance


This is for when you're a an AP lane opponent

Marks
5x Crit Chance
4x AD
Seals
8x HP Per level
1x AD
Glyphs
9x MR

PROS

Mobile Champion
Good team fight
Tons of damage ;)
Double crit chance
Wind Wall is an annoying Skill to be against

CONS

All ad team, easy for enemy team to build against
Simple to play against
Hard laning phase
Doesn't hit hard till later

So for me I've narrowed down the build paths for Yasuo to more or less 2 different paths.

1. Blood Thirster Path
2. Blade of the Ruined King Path

Key differences in the two paths.

In Path 1, you go Berzerker Boots and in Path 2 you go defensive boots.

I think Path 2 is better for beginner Yasuo players because not only is it very strong but it is also tanky enough for you to survive a long time in team fights.

Since Yasuo doesn't have a "strong" lane phase, play it safe and try to farm under tower and get good at doing that. Learn to last hit with your Q because I doubt you'll be able to farm from melee range without being harassed. Try to fight the enemy when your jungler is there or when you can all in them. Try to poke them down by E'ing through the minions and E-Q'ing your enemy and E'ing back to a creep escape. If you feel like you can all in them and try to get a lead.

Once you're ahead you can just continue to stomp, but remember to buy wards so you don't get shut down. Once you've won lane remember to roam and help out other lanes because you're mid and it should be easy.
If you aren't ahead try to get ganks from your jungler and play safe. Last hit with your Q from a distance. Don't get poked and all in when your jungler comes.

Mid game, try to get your IE and then help out your team a lot. Because with the IE and Shiv you'll have 90% crit if you're using the runes I suggested. So you'll deal a lot of AOE damage in team fights.

In Teamfights: Go for the carry. Be it the Mid Laner or the ADC go for one of them. Don't go too deep though or else you're dead. Remember you're squishy and people will want to focus you, so if you're in too deep the people will focus you and your team can't help you and you'll die. Try to get your ult off on them so you can TP to them with your ult and **** them up. Wind Wall the ADC or someone else with a lot of projectile damage so he can't harm your team and/or you.

Late game try to wait till the enemy uses their CC on your teammates so you don't get ****ed up and shut down. Go in with your ult because it is good CC and does a lot of damage.
Also late game is when the enemy team will probably have built armor to counter you. If you've build LW then post ult you get around 80% armor pen and you should be fine.