Before rating it down due to skill order, or item order, please read for reasoning, or leave a comment in what you believe first.
Lee Sin is a strong jungler, as well as lane. Lee Sin is a strong laner for both fives, and threes, but this guide is for jungling. He costs 6,300 ip, being a legendary champion. He's one I play very often, so I'd say it's worth it. Lee sin is a melee AD strong jungler. Lee Sin is just about one of the strongest champions in the game if played right with correct utility. Abusing each and every one of lee sins moves is possible, and they all come with very strong attributes. Each one of his skills has a second skill within the first one. When you use your first one, you have a few seconds to pop your next. Lee Sin's Q, "Sonic Wave/Resonating Strike" is a strong jump, you can use it in the jungle to escape, catch up, and also utilize it for juking out the enemy players. Lee Sin's W, "Safe Guard/Iron Will" is very strong for blocking off that ignite, or turret hit to even jump to a friendly minion to jump away. Lee Sin's E "Tempest/Cripple", reveals enemy unites and can slow those champions trying to run away. These are such a small bit of the many, many possibilities. With this in mind, I'll go in depth for examples, and things I've ran into and survived the impossible.
Skip to the Creeping / Jungling / Utility, if you're just looking for some tips and how to jungle.

Pros:
~ Lee Sin has very strong utility.
~ Very strong early game, still strong in mid-late.
~ In my opinion, Lee Sin is the strongest ganker in the game when used correctly.
~ AOE (area of effect) ultimate.
~ Energy champion!
~ Awesome animations and what can I say? He is indeed, Lee Sin, the blind monk.

Cons:
~ Bit squishy early game, not a problem!
~ Practice comes perfect, to play Lee Sin to his full extent, practice.
~ If you miss his Q, you can often mess up a gank.

For Marks, I take flat armor pen to clear the jungle faster, as well as taking out the champions for early ganks.

For Seals, I take flat attack speed to stack on with Lee Sin's passive, flurry.

For Glyphs, I take magic resist per level to keep some early magic resist for better sustain.

For Quints, I take two flat AD quints, and one armor penfor faster jungle clearing.

I take 21/9/0 masteries for Lee Sin. You CAN take 21/0/9 for the extra long lizard/golem buff, but you don't really need it. I take the 9 into defense for faster, more healthy jungle. I have more hp, and clear it faster with the damage reflect onto minions. I don't take the extra mastery point into the smite mastery, because the extra ten gold isn't worth it that much. I'd rather put a point for HP regeneration, or armor. The 21 offence is pretty much obvious. The extra damage output, and life steal is optimal for any jungler. The armor penetration is very strong for jungling as well as killing champions.

Skill sequence is already in the top. Maxing your Q is the best thing you can do, after you get utility from your W and E. Your E is primarily used for slowing, so one point and maxing it last will be fine. Your W will be what you put into if you cant put into your ult or your Q. Obviously put into your ultimate whenever possible, then your Q, then your W, then your E. The W's life steal is very strong from iron will, and the Q's jump and CD are so short and quick so you max it first.

Dragon's Rage > Sonic Wave > Safe Guard > Tempest
After,
1 = E
2 = W
(for jungling)

What you SHOULD have if you've completed the game with a full build (in order) should be,
Wriggle's Lantern (replace with blood thirster at the end of build) > Mercury's treads (get rid of that CC) > Trinity Force > Warmog's Armor > Atma's Impaler > Force of Nature.

Wriggles is one of the strongest items in the game, using it for the ward is very good, along with the small early game attack damage and life steal. With the ward you can do three things,
> Ward buffs, baron/dragon.
> Ward lanes.
> Use the ward and safeguard to it when stuck in a sticky situation (being in cataclysm, or safe guarding across a wall to get away.

Mercury's Treads
> Removing that CC is very good for getting out quickly.

Trinity Force
> I usually phage, then zeal, and then sheen.
> Strong with the damage output and slow.
> Overall good item for many champs.
> Sheen proc from your Burst into auto attack is strong.

Warmog's Armor
> Mid-Late game he has less hp than most champions, it's obvious that the warmog's helps a lot.
> Extra hp is very good for Warmog / Atma's Combo. (Atma's attack damage scales off of hp, good cause it also has armor.)

Atma's Impaler
> Scales off hp, good with Warmog's.
> Good AD/Armor.

Force Of Nature
> After your warmog's and not having much of a chance to go back, FON gives you the hp regeneration so you don't have to go back, and you don't have mana.
> Good sustain.

Wriggle's Lantern = Blood thirster ,
In late game, the ward isn't necessary because the support should be buying wards, and the blood thirster with just a couple of stacks will give you much more damage and life steal than a lantern. But it's more expensive, that's why it's replaced in late game.

Smite is obvious, get it unless you're going to lane. It's for stealing enemy buffs, dragon/baron, and obviously to finish your own jungle faster.

Ghost is for the extra speed. With all the jumps, flash is unnecessary.


There are some optimal spells,
Cleanse - it's okay to cast off ignite, or escape, but mercury's treads
Exhaust - Slow is good, but already comes from your E. Mostly used for the smaller damage output, but why do you need smaller damage output if it's a 2v3 or 2v1 gank?
Flash - Definitely my third choice if you don't find ghost useful.

This will be the most detailed section.

The thing that makes Lee Sin a jungle is his passive.
Flurry - "After Lee Sin uses an ability his next two basic attacks gain 50% attack speed, and return 15 energy each."
When Jungling, creep scoring, or attacking a tower, use a spell and then use two basic attacks, then use your second skill. (Q, then second time Q, then E, second time E). Be sure to get each of the two basic attacks in for jungling or tower attacking.

Creep Score

With creeping get creep score whenever possible. When going up to gank, unless it's a very good opportunity, pick up the wolves or wraiths on the way there. The extra gold is very nice.
When going into a lane, if you have a failed gank, take a couple of farm off the laner because they'll still be close to the same because the enemy ran away. The only problem with this is pushing their lane. Sometimes a good idea, not always.
While you're creep scoring, use your mana free skills, and abuse flurry.

Jungling pattern
level 2 gank
Start with Q, then go E.
Wolves > Blue buff.
Then gank.

My jungling pattern
Start E, at wolves, focus the big wolf.
Smite the big wraith, then finish off the other wraiths and get your W after the other two wraiths.
Head to double golems and focus the big golem, then small.
Go BACK up to wolves, kill them.
Go to wraiths kill them real quick and get your Q.
Head down to Red fast and use your flurry in good use, and your smite that'll be off CD and with that, you're level three with red. Gank a lane, if successful, you'll probably be going back with a first blood in your hands.

Somethings that're tips, and that you should keep in mind.

> Lee Sin goes off of energy, so you don't need blue. I personally like to get the first blue for the EXP, and it does give you CD reduction and regeneration, but it's a much better idea to give it to your AP mid. Your choice to get it first, or give to AP mid.

> As Lee Sin to clear your jungle quickly, Q through a wall, and jump to your target. It'll be a waste of two of your flurry attacks, but should be fine as it's later in the game.
*Quick tip* Use your Q probably last, because the jump scales off of how much hp the targest is missing. Take advantage of it, but it caps at 400 for minions.

> While jungling or in lane, after a gank and deciding to stay, use your W and go straight into iron will for the free life steal. It's a very quick smooth way to get your EXP back.

> If you have a failed gank, don't worry. Maybe you wasted their flash or ghost. Remember to think of when their CD is finished. Gank them again within the time of it coming back up.

> If you encounter their jungler, and kill him, or he goes back, go to his jungle, keep closer attention to lanes and steal their buffs.

> When ganking, you can either open with Q, or if they're pushed, chase them a bit, and get closer to ensure the Q shot. If you miss it, it's not a failed gank yet.
*Quick Tip* Flash initiations can be worth it. If they're about to get away after popping that flash, pop flash behind them and dragon's rage them back towards your tower, to the person helping you gank.

Utility
This is what makes Lee sin, Lee sin. I've gotten SO many kills where I pop my Q, jump to him, ult him and just kill him like that, and safe guard out to a minion to avoid more than about two shots from the turret. The quick burst catches them off guard.

Utility with your Q
> Use it to finish off people running away. Remember, it scales off of missing hp.
> Jump to minions to get away through walls.
> Use it to initiate fights.

Utility with your W
> I've been saved from ignites/tower shots with this handy tool so many times.
If you don't have any ally units to jump to, just self cast it.
> After tower diving with a quick combo, safe guard out of there.
> With your wriggle's ward through a wall, and safeguard to it.
> Use your safeguard to save your team mates, not just yourself.

Utility with your E
> Use it to slow. If you don't have red, pop your cripple as fast as possible.
> Use your tempest to reveal invisible units like shaco's deceive, or akali's shroud.

Utility with your R (ultimate)
> Use it to hit away your enemies. Remember, if it gets knocked into other enemies, it'll hit them up too.
> Flash in front and use it to hit running enemies back.
> While people are chasing you, hit the person in the front and pray it knocks up the others too.

If you have any ideas, or disagree with something I have here, leave a comment. I'll state my opinion, and maybe agree with yours.
Thank you for reading my guide.