The main purpose of this guide is not explaining every single aspect of this champion, it's to give a way in which you can carry the game and not be considered a troll.

I've seen a lot of AP YI with strange builds that don't do damage and are so squishy, so I'll try to explain in a few lines how to be PRO

With this tips I'm sure you are not having problems playing as AP YI.

I use masteries of a normal AP carry 21/0/9, as you can see above: Max damage and great sustain for laning phase.

In the case of runes, I prefer getting Magic Penetration Marks to deal almost true damage at the begining, Flat armor seals to be less squishy, AP/lvl Glyphs and Flat AP Quits to get an important AP buff.

Flash and Ghost:
Are the best spells for this champion, both can be used in offence and in escaping, plus Yi's Highlander its a great combination.

The best option is to get a Doran's Ring cause it gives you AP, mana regen and health.
You don't need to get potions or health regen cause you have Meditate .
Tips:

- Meditate is not so strong till lvl 2, so try to not lose too much HP in the first four rows of minions

- Alpha Strike is your main damage source, but it has a long CD at first lvls. Use it when there are only three minions left so the enemy champion can be hit.

- When your enemy is close to his tower and you have no minions, wait for enemy minions, get close to the enemy champion (so you can see him) and cast Q to the first minion of the row.

- Take advantage of Highlander that refreshes your CD of all your spells. You can use double meditate to tank your enemies in early game.

- You can stay in lane a long time, return only when you can get another Doran's Ring and magic penetration boots.

What you can only do in early game is to harass. Here is the main reason why AP Yi is not considered as a good champion: People say he is only good at early game. THAT IS A LIE. Lets continue:
Once you get your Deathfire Grasp you are able to deal a decent damage.

Mid Game Tips:

- Highlander is your main source of damage (refreshing CD) so try to fight only when you have it available.

- In team fights, understand that Meditate is not a spell that turns you invulnerable, it can be easily cancelled.

- Never start a teamfight, wait till any enemy has low HP, then activate Highlander and use Alpha Strike in that champion. CD will be refreshed if you get a kill or an assist, so you can cast it again to another champion, and so on. Its important to get that while highlander is activated.

- Use your Deathfire Grasp with the champion with highest life precentage before using your Q.

After getting Rabadon's Deathcap and Lich Bane you are able to kill any champion.
In teamfights:

- Before any combo, activate Highlander

- Focus the Carries of the team: Use the next combo--> Deathfire Grasp then Alpha Strike and to end a basic hit with Lich Bane effects. Most of the times you will get the kill.

- After doing that continue with Alpha Strike and basic hit, always focusing the most squishy champion.

- If you don't have CD, you can activate your E to activate Lich Bane for the next hit.

Remember:

You CAN 1v1 with Deathfire Grasp - Alpha Strike - Lich Bane

You CAN'T start a teamfight

You CAN'T use Meditate in the middle of a teamfight

You CAN chase an enemy and get an easy kill if it is low on hp

You CAN tank towers after lvl 10 with Meditate (Tip: After tanking the tower, use Alpha Strike to stop being targeted by the tower.

You CAN tower dive since lvl 6 with the help of Ghost and Flash but it's difficult if you don't know to play very well.

(if you have problems with the image above, right click and open in new tab)

Thank you for reading, hope it is useful for you :D
Good Luck, HDDiego.