That's a guide to play Lee Sin essentially as a counterjungle.
Lee,as all of the fighter jungler,can, and have to go, in the enemy jungle to steal buff and fight the enemy.
He's a really powerfull jungler, being a high mobility champion with an incredible attack power in early game,but of played all AD in the late game will be pretty useless,so i started this guide to get the highest potential from early game and still be a war machine lately.

Starting without potion you will need some life steal to clear the jungle not being always with low life. That's why with 2 quintessence of Life Steal and Lee's Safeguard / Iron Will you will be always with enough life to fight.
Starting with the Doran's Blade you will get 10 attack damage and 80 hp +5hp per hit. That means that you will start the game with 78 attack ( counting the quintessence and the 3 marks of attack damage) 80hp more then the the enemy jungler and a lot of life steal.
If you think that that's not enough to be ahead remember that you have 10 armor penetration (runes and masteries).

Get Banshee's Veil if you need high magic resist and if the enemy team is composed by champion like Kassadin, Blitzcrank or anyone else who you can troll with the shield.

Frozen Heart is pretty nice choice if the enemy have a fed carry. In combination with Randuin's Omen and Tempest / Cripple his attack speed will be ****ed in half a sec.

Locket of the Iron Solari a pretty nice item, but if your support got already it, you can skip it.

Mercurial Scimitar: how to troll Skarner, Warwick and Malzahar ( if they will ulti you, use the mercurial's passive. If they ulti everyone else in your team, kick them off).

As spell instead of Flash you can get Exhaust, if you know how to wardjump as quick as possible. But is still preferable have Flash

Ghost is a good spell too, usefull to move faster in the jungle but not as effective while ganking.

Smite: come on.... you are jungling, you don't want to let the enemy steal your buff.get dragon,baron and even their buff, don't you? so GET IT.

Start always at the bot lane side. so you can be helped by your carry and support to get the buff smitless. After that you have INSTANTLY to go and get the other one buff.

Now you can start to counterjungle, go in the enemy jungle and wait him at his buff. With your build, doublebuff and lvl 3 you are ahead, and if you don't get the First Blood you sure will force him to recall and get his jungle.

Be always sure to know the enemy buff's spawn time and go to shop 1m before,to be full life and with some new items to fight.

IMPORTANT:
-YOUR PRIORITY IN ORDER ARE:
1.ENEMY BUFFS
2.YOUR BUFFS
3.GANKS/FARM
4.OBJECTIVES

-JUNGLER IS A ROLE EXACTLY LIKE MID OR TOP, IF U CAN GET THE KILL WHILE GANKING,DO IT, BUT WITH THE KILLS YOU HAVE TO GET THE RESPONSIBILITY TO CARRY THE GAME.(if your team mate is underfed you can leave him the kill to catch up)

-IF YOU SEE A LANE LOSING SO HARD YOU HAVE TO CHOICE:
1.go and help him to get back in the game.(not the best choice for win,since it's probably the enemy will kill you too and will be overfed,but it's a nice choice to avoid flame)
2. go and make pressure on the nearest lane, so destroy the tower, get the kill and go gank the losing lane with your team mate.(that requires a high sense of team play).