Hi! I'm Chazar, a Master Yi main of the BR server.
This is my first guide so i hope to help someone. If you have any question you can write a comment, i'm free for reply questions.

Pros:
-Simple champion mechanically
-A late game threat (hard carry)
- Alpha Strike is one of the best skills in the game
-Good for splitpush
-Big damage output

Cons:
-Squishy (fragile)
-"Bad" early game
-Do not have any cc and vulnerable for them
-Predictable

Primary

Precision

Lethal Tempo

My main jungle build is full speed attack and lifesteal, so lethal tempo will work well with that build.

Conqueror

I usually pick conqueror against tanks, because conqueror gives armor penetration.

Triumph

Talking seriously, triumph is the best rune in this line.

Legend: Alacrity

Rune to boost attack speed.

Coup de Grace

Rune to get better ganks/1v1's.

Secondary

Sorcery

Celerity

Gain more movespeed for better ganks with ult, get into tfs, and so forth.

Water walking

Good to tf's and 1v1's in the river.

Domination

Sudden Impact

Letality to Alpha Strike do more damage.

Ravenous Hunter

This rune is good to get more sustain to tf's and 1v1's.

Inspiration

Future's market

Future's market is a good choice against champion with a good early game, like Lee Sin, to get a early items more fast than normal.

Cosmic Insight

Works well to get more cdr to your Alpha Strike, items and summoner speels. (Good with Ghost)

- Master Yi Alpha Strike can be used to dodge skill shots and auto attacks from enemies and jungle monsters.

- You can use Alpha Strike to change target like, for example, from support for the adc.

- Alpha Strike can be used to follow enemies in teleports, advances, and so forth, like flashes and Yasuo Sweeping Blade, for example. So don't start a gank with Alpha Strike, wait until they use their Flash then Alpha Strike.

- Alpha Strike can be used for cancel hits of towers.

- You can use your Meditate for reduce damage (When Master Yi uses Meditate, he gains damage reduction) from auto attacks and abilities (like Lee Sin Sonic Wave) and to reset your auto attack.

At this stage of the game, you should not force trades if you do not have at least your Skirmisher's Sabre - Bloodrazor, but if you see any viable lane for ganks, do not excite. Always keep an eye on map.

At level 6, just with Recurve Bow and Skirmisher's Sabre, Master Yi can solo any drake easily, so if you will do, make sure that the enemy jungler and the bot lane it's not near. You can dodge the dragon knock back with your Alpha Strike, but you have to catch the right moment.

In these parts of the game, you should focus more on objectives, like drags, towers, etc.
In tf's, wait for someone from your team to start, if you start an tf you'll probably take all the cc's and Master Yi is easily stopped with cc's. Always focus on the enemy squishy target or the more important target, if your team kill him, you go get a 75% cd reset of all your abilities, focus on the next important/squishy target and so forth.

Thanks for reading!