This guide is aimed at people who are either new to LoL as a level 30 or new to Lee Sin. If you are new to LoL as a game, I advise you play easier champions to learn the basics of the game itself before trying out harder champions.
If you want to go from beginner to master with Lee Sin, this guide should help you get a foothold onto that path.

If you don't know how to start playing Lee, this guide should get you started.

After some thought i decided to work on this guide over time adding more and more detail to help people go from a complete beginner to a master.

I will try to separate parts going from Beginner - Novice - Intermediate - Expert - Master.
In theory this guide will eventually allow anyone to work towards being 100% confident in their abilities with Lee Sin.

Hello, welcome to my guide to Lee Sin. This will feature very basic gameplay mechanics and idea's all the way up to higher elo mindsets and skill.
Before you get started reading through this guide i suggest you think of these things:
Are you confident in your abilities in LoL overall?
Are you able to manage mana efficiently? (This will come in very handy as energy is used MUCH quicker and if you do not manage it properly you will fail)
Can you react quickly?
These are a few basic skills you will need to start your journey to mastering Lee Sin.

I am by no means a master Lee Sin player. While i may be able to perform well with the champion, i still have a lot to learn and master myself.
My highest rank is Platinum 4 on OCE so I still lack a lot of overall skill in league, HOWEVER, i am confident in my abilties with Lee Sin to give people a jump start into playing the champion.
When reading through this guide please keep in mind, ALL of it is aimed towards BEGINNER Lee Sin players. This guide is a absolute base guide that you can follow to get started, this is not aimed towards anyone with over 50 games of Lee Sin, or at least level 4-5 mastery with him.
You should either never have played Lee Sin before, or be level 1-2 with him for this guide to really help you.
Anyone with higher experience CAN benefit from this, however my guide assumes that you don't know anything about Lee Sin and cannot effectively work around different scenarios.

As said in the notes, the runes are good enough to get you through your games.
The AD will let you farm easier and let you do more overall damage early.
The armour will help you stay alive in the jungle and versus enemy ADC's.
The MR is in all honesty just there to round out the cooldown reduction, it does however help, even in the slightest, defend yourself against AP champions early.
The 5% CDR will make quite a difference in your early clearing as it gives you more access to your W lifesteal and your Q damage, it will also help later getting your R up faster to gank effectively even more.

Savagery will help you clear jungle monsters.
Lee Sin doesn't benefit from mana so cookies are half useless to him, the same goes for Runic Affinity, Lee Sin doesn't really need blue buff, nor does he take Rift Herald and you should be aiming to finish games without needing Baron.

Lee Sin's Q is an execute so taking Merciless helps you take out enemy champions and surprisingly mana regen is useless on Lee.

Anyone with over a thousand or so games can tell you, the times that you die just after killing someone happen more often than you think. Ever wonder how the enemy JUST lived when you had red buff ticking on them? Ever wonder why your ignite left them on a single digit? Dangerous game CAN give you that tiny amount of HP you need to survive that one minion auto, that last tick of ignite or red buff.

Taking intelligence is really a must because lethality on Lee Sin isn't THAT effective. The 5% CDR from your runes plus this gives you 10% CDR at level 1 and assuming you do well, by level 6 you will have 20% CDR getting your ultimate up VERY quickly to keep coming back to bot lane.

Thunderlords is really a must for Lee Sin in most cases, if you are ahead enough a simple Q R Q AA will be enough to kill an enemy ADC, adding the thunderlords into that seals the deal.

Sorcery adds even more burst to your kit, Expose Weakness helps your team do more damage to your enemies, Vampirism gives you even MORE sustain in the jungle and 1v1's, and you can swap Double Edged Sword for Bounty Hunter depending on your general play style.

You can take Courage of the Colossus as it works very well with your ultimate, if you are going to be building tank or having some HP items, taking COTC will allow you have a point and click shield in teamfights.

Your items will change depending on the game, more MR if the enemy team has more AP champions and the same for armour for more enemy AD champions.
The more comfortable you get with Lee Sin the more items you can begin to use, for example, buying a QSS will help ensure any CC used to stop your Insec will be useless, the same goes for Edge of Night.

Again, as you become more experienced with Lee the more comfortable you will be with building AD, my games with Lee usually have me building AD rather than tank because i am comfortable building and surviving with that.

Try to build accordingly to the enemy team AND your team, if you need a TANK, then get your warrior ASAP and build tank, Lee Sin can be played as a tank, you don't need to make any fancy plays to make Lee Sin work well.

The way you max your abilities changes in most games.

If you are wanting to get a level 2 gank and your teammate(s) have CC then you can take W to get into the action faster, but you also lack a slow, the same for getting E first, you have a slow but you take a little longer to get into the action but you might burn a flash or two.

As with most champions level your R at 6,11 and 16.
The flow of the early game will change how you level your abilities, if you are ahead you will want to max Q as it's your main ability.

If you are having trouble bursting enemies and getting into longer fights, maxing W might help you as a lower cooldown, bigger shield and more lifesteal can give you the lead you need to win a 1v1.
And if you are ganking 24/7 then maxing E wouldn't be a bad idea as the slow increases each level to a substantial amount.

However at the end of the day, the more you play Lee Sin the more you will become comfortable with his abilities and you will form your own playstyle with him and maxing your abilities will work accordingly.

Smite as you will be playing Jungle, do not try to play Lee Sin top until you are comfortable and understand his strengths and weaknesses.
Flash should be taken all the time as eventually you will be able to utilise it in your combo's.

Please, PLEASE, do NOT play Lee Sin in ranked until you are AT LEAST at 50,000 mastery point on him.

Lee Sin is one of the hardest champions to play unless you know how to use him, you will be flamed by your team for not being able to ward hope properly, for not being able to either Insec properly or even at all.

Lee Sin is meant to get your team ahead early and finish by 25 or so minutes. If you CAN'T do this, do not force your team to play with dead weight.

If you find yourself dying more than 5 times in 15-20 minutes, do not play Lee Sin in ranked. If you can't gank effectively, do not play Lee Sin in ranked.
If you cannot invade the enemy jungle effectively, do not play Lee Sin in ranked.
If you cannot Insec, do not play Lee Sin in ranked.

Pro's -
High damage output means less punishment for missing Q's and gives your team the ability to work off the damage you deal.
High early game damage and utility to get himself and his team ahead to clean up a victory easily.
A great kit to play a range of playstyles from Assassin, Fighter, Tank, Support.
Surprisingly "easy" kit in general, Lee Sin is a hard champion to play when bringing in mechanics that require skill and timing to pull off, he is however an easy champion to pick up if you can use him at the most basic level.

Cons -
Lack of skill but more importantly CONFIDENCE, will lead to mistakes, deaths and losses.
A lack of confidence will lead to many opportunities being wasted, eg: A kill on a carry, an objective, an allies or your own life.
If a lead cannot be created early, Lee Sin will fall off late game.
REQUIRES early game pressure to work effectively, people NEED to be afraid of you in order to get ahead.
VERY easy to mess-up combo's if a single part is missplayed, eg: Ward hop didn't flow, missplaced your ult, didn't expect an enemy CC or gap closer.

When jungling with Lee Sin it is essential to manage your passive.
Your passive Flurry, gives you 40% attack speed for your next 2 auto attacks and returns 20 energy on the first auto and 10 on the second.
Making sure you get two auto attacks off before using an ability or it's second use will determine whether or not you clear the jungle fast enough to get an early gank off.

Start at whichever buff your bot lane is closest to, with the changes to smite buffs you need them to help you clear the buff for an easy level 2.
Generally start Q, the damage helps clear it faster and gives you the execute damage to work with your smite. If for some reason you don't get a leash, start W for the sustain.

If you started red go to Raptors and kill the small ones first, make sure you manage your passive since the extra Raptor being added makes them even more deadly in early levels. Getting

Take a look at your minimap and see where people are, if you immediately see that mid or bot lane is being pushed to their tower, pan over to see what's happening, if the enemy laner(s) are on your half of the lane, make your way over to a bush to try to get a gank off.

Assuming you CAN get a gank started, ping to let your laner know you want to gank, then ping On my Way to signal you're going in.

This is the key part to ganking with Lee Sin, you will want to use your Q immediately but WAIT. If there is a clear shot, yes throw your Q out and recast to go to them, auto twice and if they use a gap closer ability use your trinket to ward hop towards them and try to force a flash.

If you can't get a clear shot, walk into lane and auto them, if you got E level 2 use it, auto twice, E again auto twice. The key to early ganks with Lee Sin is to blow their flash. Most people will try to juke your Q but hold onto it. Get them lower and lower while they waste time trying to juke your Q and once they flash throw your Q out if they are low enough to kill, if they don't have Barrier, Exhaust or Heal re-cast your Q and auto if needed for the kill.

This is a very messy explanation of how to gank level 2, i suggest you play some games and try this to get a better understanding of how to do it, as theoretical situations aren't as good as literal situations. In a real game you will be able to tell, okay i can kill them if i land my Q OR, okay i got their flash i'm going to go farm as i can't either land my Q due to minions or they have too much HP.

Generally you want to be farming and ganking, but the rule you must ALWAYS follow is NEVER try to gank everyone. You simply can't.
If you try to gank every lane you will spend too much time missing kills and farm. Pick a lane you want to get fed and gank them. If top lane can solo then leave top alone and focus on somewhere else.
If bot lane is already 0/6, ignore them, theres nothing you can do at the moment to help, get your mid laner fed so they can 1-shot the enemy ADC.

Your E and Q reveals stealthed targets.
They do not reveal traps like Shaco boxes or Teemo mushrooms, but they will however reveal those champions themselves.
If you know Teemo is in a bush simply press E to reveal him, if shaco or vayne just went invisible and you saw the ghost they created, press E nearby as the range is large enough to snag them.

Your Q is great for vision in a tight space, if you are weary of an enemy, simply Q into a bush to let you know whether or not they ware in a bush.

Your W works with most champion created abilities, you can ward hop to Mushrooms, Boxes, Thresh Lantern, Heimerdinger turrets, Zac blobs, Mordekaiser ghosts, Yoricks ghouls, Tibbers. Keep in mind, you cannot ward hop to blue wards

Try to assess who needs what.
Is the enemy assassin ALWAYS targeting your ADC that you need to win the fight? Try to peel for your ADC using W shield, E slow and R knockback.
Do you have a Yasuo? Simply ulting someone will proc Yasuo's ult to allow him to follow up.

Do you have a Caitlyn? Try to ult people out of the way so she can hit her ultimate on a low target.

Do you have someone with hard CC? (stun, snare, knockup) Wait for them to use it for an easy Q, Alistar is very good for this, letting him engage with a W Q will get a gank working much easier.

Are you facing someone with a stealth? Your E reveals stealthed and invisible enemies.

Your ultimate can simply be a tool to get someone in range of something to let your team finish them off.

This is my first guide, i apologise for the lack of depth i may have in this. I simply want to give new Lee Sin players a place to start.
There are hundreds of guides and players to follow that will work much better than mine, but if you think this helped you in anyway, that's enough for me.

If you have anything to add that would help make this a better guide please leave a comment for me.

My biggest tip for improving with Lee Sin, is just play him. Ignore the people who try to bring you down, to uninstall and never play Lee again. We all started there, even Insec himself started where you are today. Play Lee and gain confidence, if there's one thing that will hold you back, it's being afraid to make mistakes.

Play Lee Sin and learn from what you did wrong and improve yourself.

Thankyou for reading.