Hello OnlyUseMeHess here with my first heimerdinger build. This build is great for farming and creating magical acts against enemies... what i mean is late game you will be able use your abilities to tuna melt your enemies health in a matter of seconds. You will counter other mids with early game magic resist with the chalice of harmony.Fun champion to play all together and is underplayed by summoners.Since Heimerdinger is a unique champ i'm making a guide for all in search for help.

Pros
Great Pusher
Unique
Fun to Play
rarely gets banned
rewarding combos
funny moments

Cons
Low movement speed
Hard to master
Weak AP ratios
Squishy
Mana starved

You should be using 9x Greater Mark of Magic Penetration for late game when tanks get magic resistance
You should be using 9x greater seal of replenishment for the early game mana regen so you don't have to recall early game and miss out on exp then be underleveled throughout the game also since Heimerdinger is mana starved. You should't take mana regen per level since you will get a Athene's Unholy Grail
You should be using 9x Greater Glyph of Ability Power or Greater Glyph of Scaling Ability Power whichever you have already or you prefer.. Personally i like the early game AP.. I don't recommend get cooldown reduction runes since UPGRADE!!! gives you cooldown reduction and you will be getting Athene's Unholy Grail.
You should be using 3x Greater Quintessence of Ability Power for the AP and I've also faced a Heimerdinger who used Grater Quintessence of Fortitude so Heimy isn't squishy early game also for the survivability when runnning around your turrets.Which ever you prefer.

Get Doran's Ring right away so you get mana regen and AP.Getting Chalice of Harmony gives you advantage early game against other mid champs with the magic resistance and getting mana ragen is always a plus. Upgrade that to a Athene's Unholy Grail which is a great late game item. Then your first big item should be Rod of Ages to get the stacks on it which occur every minute 10 times.This is a great item so Heimerdinger isn't mana starved has more survivability. The last item i will go in depth with is Zhonya's Hourglass which is used when you are surrounded by your turrets taking damage by the enemy then you pop the hourglass' active and let your turrets go to work.Obviously get armor/ap items if the enemy team is AD and Magic res/ap if there AP
Other items are self explanatory...

H-28G Evolution Turret-Max by level 9, this is used to farm early game get those minions to purchase your items as quickly as possible, this is your main damage output, great kiting skill and always be near these turrets they are your defense too, place these down to help take down turrets.
Hextech Micro-Rockets- max by level 15, late game this does a ton of damage if you combo with your E, for squishy champs this can be a pain to go against Heimerdinger although the rockets should without your aim.Great for team fights and for kiting, great for harassing you fire 3 unless with upgrade you fire 5 rockets that hit the nearest enemies or monsters, if you face a off tank mid i recommend getting CH-1 Concussion Grenade maxed 2nd.
CH-1 Concussion Grenade-Very underestimated skill, used great when your surrounded by your turrets and an enemy comes near and you stun them then let your turrets do the work, also great against ad carries since when you blind them they do no damage. When you learn how to use this it is a turning point in a battle or a 1v1, lastly good for vision in bushes and dark spots on map.
UPGRADE!!!-Heimy's ult which benefits you by:

Healing turrets and giving them slows
Concussion Grenades travel faster
Shoot 5 rockets instead of 3
Cooldown Reduction of 10/15/20(Passive)

Useful when your team is running away or your in a team fight, great for kiting, counter ganks
Use once about halfway through teamfights when the enemies tries to make a last second escape you slow them and get the kill.
Techmaturgical Repair Bots(Passive)- Heimerdinger gives himself and nearby allied structures and units 10 / 15 / 20 / 25 health regeneration per 5 seconds.
Diameter: 800

Flash is a great spell for Heimerdinger because of his low movement speed, also if you are getting attacked next to your turrets you flash onto the other side of them and then the opposing champion will follow you and will get kited.

Second slot is your personal preference

Ignite is the normal spell for an AP carry to finish the enemy champion if they survive your burst or stop healing from a Dr. Mundo or a Volibear from healing.

Teleport you can recall get health and mana or buy an item then teleport to your lane or if you push your lane you can teleport to another lane and help your team out.

Heal Bait the enemy champion by having low health then heal turn on them, good survivability

Heimerdinger rarely gets banned in ranked play,he is a great team player and carry, great pusher,easy to farm, has many moves to have vision across the map.

Use your turrets to Slow enemies when your team runs away spam your W and E
Use your E to give vision in bushes
Place turrets after your team initiates

Dont initiate
Dont support/Jungle
Keep your distance so AoE spells dont kill your turrets
Use Concussion Grenade on enemy AD carry to blind them
Put down turrets during fight
Spam W and E

Max turrets early game to farm and push you will be so farmed have a ton of money during game

Hope you guys enjoyed the guide give positive rating if you enjoyed
Please place a comment before giving negative rating so i can explain or fix the problem
Heimerdinger isn't OP it's just that your good
And please dont troll vote like other people have in the past