Guide not really for the public but for my friend who wanted to learn Heimerdinger. I will maybe make it more friendly and such for the public at a later time, but I'm quite busy right now and so this is currently really rushed and really rudimentary.

This is how I play Heim and I really couldn't care whether you (as some random reader) agree with what I do or not.

FYI, my build is adapted from Dan Dinh's Heimerdinger in WCG 2010 against CLG. Search it up on Youtube; I'm too lazy to link it here. It's different from most other Heims around mfire.

Runes
Magic Penetration, Ability Power, and Mana/Mana Regen are priorities. You could also get a bit of flat health on quints if you find you're not tanky enough.

Masteries
9/0/21 or the 6/0/24 I have above.

Spells
Flash is a must.
Beyond that, it really depends on your opponents, and the rule is anything goes except for Revive (because no one gets that, and for good reason).

Cleanse for CC nuts
Exhaust for fastholes
Ignite for Soraka
Clarity if you're still learning the ropes and don't know how to save mana
Clairvoyance/Rally/Fortify/whatever if team support is inadequate
Ghost if you're still clueless. (Flash + Ghost = successful escape)

Earlygame: QWQWQR or WQQWQR
Lategame: Ulti when possible > Turret > Missile > Grenade

First earlygame skill order to put a turret up before the creeps come for sight. Second order comes from the Dan Dinh build, allows you to harass an enemy from level 1 and hopefully get a level advantage. After level 6, you want your turrets for pushing so max that first, then missiles. Grenades are mana costly so they are gotten later.

1.) Heimerdinger loves solo lanes, but unfortunately this means he has to run around a lot and he might get hit a lot. Start with boots and health potions to help you run away from harass and heal up when you do get hit.

2.) Heimerdinger loves spellspammy, and most players do it right from the start! Pssh, be a good player and learn to conserve and use your physical attack instead. The only mana regen you should need is a Meki Pendant.

3.) Then you can go for a really early Kage's Lucky Pick, which is a really inefficient source of AP...until 13 minutes later when it has effectively cost 0 gold. (You're going to be selling it, btw; it's 25 minutes if you are making it into Deathfire.) So you really want to get those 13 minutes in as quickly as possible.

4.) Make your Meki Pendant into Xia0_RaiN's favorite AP Item, Morello's Tome. You have mana regen for spellspammy, CDR for spellspammy, and AP for more powerful spellspammy. So when you build this, you should start the spellspammy.

5.) Don't forget to build into Sorcerer's Shoes!

6.) Defensive item of choice is Soul Shroud. More CDR, more Mana Regen, and a decent amount of HP.

7.) You've made it to this step without losing the game! N1! now it's time to head up to the big items. My luxury items of choice are Abyssal Scepter, Zhonya's Hourglass, and Rabadon's Deathcap. Build them in any order, and feel free to even start on another recipe without completing the first one. I personally like to get get 2x Blasting Wand first before building into Abyssal and Rabadons, then get Hourglass last. But it really depends on the game and how much gold you have when you decide to Recall.

8.) By 14k gold, you've (hopefully) won the game. Otherwise, you're not doing it right. (HINT: you're fking Heimerdinger...push harder.)

I'm supposed to summarize my guide here. But legit summaries...who reads them? The real content is in all the other sections above.

GLHF on your techmaturgical endeavours.

Xia0_RaiN