Welcome to my first guide.Here i will show you how i build my Master Yi.
Enjoy and have fun reading it :)

PROS:
-GREAT damage dealer
-Very mobile
-Farmer
-Pusher

CONS:
-Squishy(especially early game)
-His largest weakness(Blinds and Stuns)

As for the runes, you can take the glyphs for the AS instead of crit dmg. But in my opinion it is better to take AS glyphs. It is your decision:)

I use standard DPS masteries 21/0/9 with improved Exhaust and Ghost. In Utility i took health and mana regen and a little bit reduced dead time .

There are much waves to build Master Yi. But i concentrate on AD,AS and Lifesteal.
At beggining buy Boots,2x Health Potion and 1x Mana Potion.
At your first recall you should have enough gold to buy Berserker's Greaves and Dagger.Your next item should be B. F. Sword. At you next recall buy Black Cleaver and Vampiric Scepter.At this point you should have good AS good AD and Lifesteal!!! When you get money buy Bloodthirster .Next item you should collect gold for is Infinity Edge. After it take Phantom Dancer and Frozen Mallet as fast as you can.Now you should buy elixirs and make people leaving game :DDD

As you can see first thing i max out is Wuju Style. I do that because of it's crazy damage it gives and when you activate it... you get DOUBLE WUJU STYLE man that's good!!!
I max Highlander as any other ultimate.
Alpha Strike or Meditate ? Alpha does great damage but Meditate gives you MR and Armor while doing it. My solution is: after lvl 11 leve like this: QWQWQW... or WQWQWQ... your choise.

Best Spells for Yi:

Exhaust-Opponents try to run? This is Spell for you,it slows the enemy,reduces his/her movement speed and damage,what else do you need???

Ghost-You could kill enemy but he ran away? Or you get killed becasue of your speed?
Ghost is great Spell either for chasing or running.

Alternative Spells:

Flash-Great Spell(you can flash trough walls)for running and screwing an opponent

Ignite-Great to finish the enemy that rans away...

Cleanse-Removes stuns,blinds,all that you hate so much.

Spells NOT TO USE:

Fortify-Leave that to tanks and supports.

Teleport-You are very fast so you shouldn't take this.

Clairvoyance-Why????

Rally-You've got your Wuju Style.

Clarity, Heal-Good early and middle game but useless in late game.

Revive-Just...no

Smite-You're not a Jungler

When ganking: (use Ghost if needed) E --> Q --> and let your autoattcaks finish the rest if the target is fleeing use Exhaust and use Highlander to finish the target.

Thanks for taking your time to read my guide hope you will enjoy killing your enemys :PPP