MOST IMPORTANT INFORMATION IS THAT YOU TRADE OUT WRIGGLE'S FOR BLOODTHIRSTER!
Now that that is out of the way, welcome to my Lee Sin guide. Lee Sin is best played as a jungler, so I have made a jungle guide for him. If you have a question about the guide, chances are that it will be answered in a later chapter.

Lee Sin is a jungler, so he takes smite :D. The flash is used for three reasons:
1. Positioning for Dragon's Rage
2. It is better than ghost for most junglers with very few exceptions
3. If you invade the enemy jungler, chances are they took flash and your flash could lead to a first blood if your Q and W are on cooldown

The reason why I use these masteries over a 9/21/0 build or a 21/9/0 build is that Lee Sin has built in life steal and we will be getting a Wriggle's which will later become a Bloodthirster, so there is no reason to go down into the life steal on the offensive tree. Also, all that is useful for a jungler in the Defensive tree is the armor and minion damage return, so you do not need more than 12. Plus, this allows for the Runic Affinity in the Utility Tree, which is a vital mastery for any jungler.

Because of the AOE damage on Lee Sin's E and the attack speed lower that it gives, it is the best spell to take at level one, unless you are starting at golems (which will be explained in the different routes chapter). It is vital to take Lee Sin's W next because of the life steal and the armor on Iron Will, and the shield itself. I max Lee Sin's Q after this because it gives the most damage out of any of Lee Sin's abilities (besides Dragon's Rage), especially if the target is low health, as Resonating Strike deals execution damage. I max W after Q, but before E, because of the shield and the supporting element that goes with it. E is maxed last because the slow is powerful enough at rank 1 for awhile, and you will not be using it for damage.

I use flat ad and armor pen marks because Lee Sin's passive allows for straight damage on his marks. Armor seals and MR/level glyphs are standard on all junglers. Now I take the movement speed quints because it is extremely useful for ganking, and it allows for level 2 ganks if you go for red buff then level two ganks.

As I said in the beginning, it is imperative to sell Wriggle's and get a Bloodthirster after you get a little more tanky. The rest of the build is focused on being a damage-soak, Even if your team already has a tank, it ALWAYS helps to have another person to take the damage that would be going to the carry. Also, the Wriggle's Lantern is a very important item to keep until AFTER you have gotten most if not all of your tank items, because it allows a good amount of life steal and armor, but more importantly the chance to instantly kill a minion. The Wriggle's also helps with warding and Baron/Dragon control. The rest of the items on the build are geared toward being able to stay in the fight for a long time and to be more of a useful teammate than someone who walks into a fight, kicks someone, and then dies.

The usual path for Lee Sin is wolves -> Blue -> wraiths -> golems -> Red -> wraiths but if you want to start with boots and 3 health pots and want to go for a level 2 gank, you can take red and either counterjungle or gank mid and/or bot straight away. Lee Sin has probably the most number paths open to him which can be successful for him, because of his incredible utility.

Lee Sin is meant to be a damage soaker with some damage, and possibly the person that sets up kills, like a Janna. Lee Sin is not meant to carry games. Lee Sin cannot carry games. Lee Sin is most comparative to the person who the guy who shouts at you during a sports event. Lee Sin is the kind of champion that helps your team to win by putting the opponents behind, not necessarily bringing your team up. That brings me to the fact that Lee Sin is a control jungler, like Nunu. If Lee Sin is level 5 when everyone else in the game is level 18, with the exception of their jungler who is also level 5, then Lee Sin has done his job well. Lee Sin therefore, as a control jungler, specializes in counter jungling, counter ganking, and because of Lee's match-ups he specializes in killing the other jungler.

If you take nothing else from reading this guide, take these two things:
1. Sell Wriggle's Lantern late game for a Bloodthirster
2. Lee Sin cannot carry games, but he certainly helps his team to win
Also, because of Lee Sin's diverse jungle pathing, anything as far as path and starting item can change depending on the enemy jungler and if you think that they might try to invade your jungle. Cloth armor is the safest way to start, especially if you get counter jungled, but boots or even a doran's blade are both very viable starting items depending on the enemy jungler.