Hi everyone!
First of all , this is my first guide. I've been playing Heimerdinger since ... About lvl 15 , I think... So I reached a high mastery level (maybe not...). Now, on to business. I've seen many Heimer guides, and, personally, I find most of them useless in-game.
Please don't be too harsh with me, I have little to no idea of how to write a guide... I just want to give my opinion, and see if anyone out there gives a coin for it .

Okay, so most people hate Heims, because he is OP. And it's true. Especially with good builds and skill set. People usually don't know how to deal with him , and get nervous with his turrets, making mistakes and losing. Bad thing.Many consider him a Noobchamp, because of his turrets. That's ... not actually true. He can't nuke you with one skill and then go away. Besides, his R opens up tons of opportunities. Each action leads to a different outcome. Heimer is truly a surprise if played well. No noobchamp offers such a variety...
Another thing about him is the humongous farm he can get, or how he can kill so much, even after he dies. Not only that, but his turrets can act as wards, his grenades , on Ult, can obliterate clustered enemy champions, and missiles dish out a helluva damage.

Let's start with summoner spells. Ignite is quite obvious, you don't want your enemies to escape. If you happen to die, at least take them with you. Works wonders with R+q and Red buff.
The thing comes with Exhaust. Most people criticize it, saying Flash would be better. Wrong. Exhaust will stop any incoming gank, allowing you to score a double, if not triple , kill. It makes your turrets a deadly cage, few can escape once Exhaust is upon them. Besides, having no boots later on, it can help you escape or catch up.
Now then , on to the game itself. Pick that Doran's Ring, some cookies(upgraded Health Potion), and rush as fast as you can to Mid. As soon as you get there, pop a H-28G Evolution Turret. Jungle will probably ask for help... But you must assure you have three turrets up and ready to farm as soon as minions appear... So you better let someone else help. Not that you can do much at lvl 1, either.
The position of the turrets depents on the situation. Mostly, however, the best choice is a triangle, like here(btw, not my pic, not my account, sorry if I do something wrong... :O):At first, attack with basics your enemy, and don't let him get close to your turrets (This doesn't mean that you have to get yourself killed, though).
As soon as you get your W, the Hextech Micro-Rockets, start poking the enemy. They don't do enough damage just yet, but , one by one, they will sicken the opponent, get him stressed and weak.
Stay in line until you have enough to get the Needlessly Large Rod . However, if you run out of mana/potions and the enemy is not pushing too hard, rush to base, fetch 5 cookies, get your mana back up, and get back to the front! Don't buy anything else, however. That big rod is essential!!
Assuming you got the neccesary items, the core ones, let's get down to the beating part!

The H.S.W.S.E.G.C.Y.W.B.F. is a short guide (contrary to what the name points out...) that will teach you how to nuke the hell out of them!
Turret strategies:

The Berm-heim-triangle: Put the turrets within range, but spread enough (as in the image above). This ensures that any gank will be stopped , will protect you if the enemy jumps on you, and , furthermore, maximize the farm.
The 'You shall not pass!': This one consists of forming a line with your turrets. It's not an usual configuration, but it serves well for things like blocking the retreat of an opponent, allowing your teammates to kill him(or even dive under tower), or to mark a line that protects and warns you and your allies of incoming threats (ok, this one I only used twice in all my life, don't really rely on it).
The persecutioner: Keep the three of them in store, and rush one escaping enemy with W+E. Once slowed, get close to him/her and pop the first turret in front of him. If he/she manages to escape, persecute him/her with Exhaust+ the remaining turrets. Few can escape this.
Tower-breaker: This one depends a lot on the tower's health, the power of your turrets, and if there are any enemies around. Make sure you CAN destroy a tower before doing it, or else you will waste your turrets. Wait for minions to engage the building, and surround it with turrets. If enemies are coming to save it, pop your R+Q. This will likely destroy the tower and scare off any enemy, allowing you to retreat to safety, while Ult. cools down.
Ultimate Doom: This is about your Ultimate Turret. The trick with it is to save it until many adversaries attack you. Get inside the triangle, pop your Bad Mother-Shooter, embrace the Zhonya's Hourglass , and enjoy the fireworks. Of course, you can use the big turret without many enemies, or without turning into gold for 2 seconds... It depends on the situation

Turrets done, let's get to Rockets:

Pokey: You have no idea how annoying this can be. Just sit back, blast your enemy, make him/her waste potions, and enjoy the show. Chances are, when the fight begins, your opponent will have less health than you, and no way of escaping your clutches.
Seeker: Wait.. Wait.... THERE! POP! BOOM! DEAD! MWAHAHAHAHA! ... That's how you should react when hunting with your W. This is later on, when you have enough AP to kill an enemy with 1/4-1/2 of health in one burst. This tactic implies that you have a secret weapon, ready to kill anyone. It's good for sniping out , destroying hiding cowards, and obliterate those that try to escape.
Deathwave:You see that guy under tower? Wait, no need to turret-dive. Just R+W and *BEWM!* , he's gone. Of course, you better have enough health to come out alive, and make sure HE has enough health not to survive (WUT?) ... If you are lucky enough, you can use this to kill several wounded enemies staying together.

Now then, what's left? Oh yeah! The terribly underestimated HOLY HAND GRENADE OF HEIMER-OCH!

Freeze!: What's this? Jungler tries to gank me? Meh, whatever, I'll pop a grenade in his face and let the turrets shred him to pieces. This is meant for this. Stunning anyone dumb enough to get into turret range, probably getting him killed. This can be used together with the Bad Mother-shooter, but don't expect it to shred...It will just turn the poor thing to ashes.
Toss and Kill!: Assuming the grenades have AoE, and same range as the missiles, you can use them to stun, or slow, at least, those that are just in the range, but not close enough to get sniped by missiles. Many, as I said, underestimate the grenades. A good grenade can turn the tides of battle.
Triple Boom!: What's so great about this? Well... Tremendous damage, stun, slow ,and AoE... That's what makes it great. However, using it in teamfights is not that a good choice, since the Big Turret lasts longer... Use it to blast into the oblivion enemies out of your range (Yes, it's one of the many sniper qualities of Heims). It can also help your team secure an Ace, killing the escapists.

Many of you will ask why there are no boots. Answer is simple: not necessary.
The enemies will be greedy and will come to you, so you won't have to catch up with them. And if it's the case, Heimer has many ways of killing escapists.
Now then, about the other items. Zhonya's Hourglass is essential. You need it in order to survive under your own big turret, and score kills. It will save your butt, even when it looks like you are about to die. Trust me. Dont you ever forget to take a Zhonya.
Most items are for AP. With 600-700 it should be enough to destroy your opponents. Of course, if you want more, there are many options to pick from...
Why the tank build, though? Heimer is very squishy. You will need extra health in order to survive. With Guardian Angel, for example, you will ressurect. It is, however, a Just-in-case. Only take it if enemies get fed and kill you even with Zhonya's Hourglass.
This build ensures your safety. You will get more AP and Health/defense than most champions in your match, making you truly OP. Who doesn't want that??? Your 1/6 will be like their 1/3!! This, plus that damage, will make you immortal.

I won't lose neither YOUR nor my time with explaining Heimerdinger's pros and cons, counters, abbilities... There are tons of guides out there that will tell you these details. Why repeat over and over again what you already know??? I just want to show you , the reader, something innovative, a truly OP meta-breaker.
This definetly is NOT the best guide. You won't find many pictures, many links, and lots of additions.. Oh no. Just what I want to tell you. Sorry, if you were looking for a shiny, beautiful guide, then I must've dissapointed you. I ask for you to pardon me for wasting your time, if I did so...
Now then, pleasantries apart, I can't think of anything else to say (might be because I had the 'brilliant' idea of making this at fking 1 AM!!!), so... I do hope you enjoyed this! I sure did! Tho...*yawn* ... My head is starting to hurt... Meh, whatever...
Thanks for reading this useless stuff, it makes me happy! Thank you, oh , unknown admirer, for basking in my ethereal glory!