Okay, so this is obviously not a guide.
I don't think I'm skilled or experienced enough to tell anyone how to play.
Though I don't mind giving direction in game. ;)
This, as you can see, is a build. More specifically, my build, or the build I've been using with lots of success.

Couple of notes:
Above, it claims the AP at 18 is 430, this is incorrect; this does not take into account the 30% from the deathcap or the 3% of total mana conversion from the archangel's staff. AP at 18 is usually between 650-800.
I only mention this because the Deathfire Grasp's 30% of total health + 3.5% for every 100 Ability Power. :)

And finally, and most importantly, the combo for team fights.
Keep pounding with rockets, baiting (while staying in your turrets' range) until they hop on you. (don't over extend, even a little; if you're playing correctly they already want to target you.
Then, it's on.
1.) Activate Zhonya's Hourglass (its the 1 key for me)
2.) type /l or /d... I like /l but it's up to you.
3.) spam F (flash) to the safest spot near your turrets
4.) UPGRADE!!!, ulti, R
5.) Rockets, W
6.) Grenade, E. Try and hit the AD carries or a cluster of them
8.) Activate DeathFire's Grasp. Again, select your target wisely.
7.) Rockets, W

After you flash,(step 3)keep moving around while staying in range of your turrets.
Heimer shines most with a little patience.
So be patient.
Also, remember that Heimer's turrets fire 50% faster for the first 6 sec when you place them. ^.^ this is useful information.

Legitimate questions will be answered. .. Probably...I am pretty lazy.
Stupid questions will receive no response but will likely provide me with entertainment.

Haters and Trollers...you'll do what you do.
Lovers show some love.
But what I'm seeking here is a little feed back.

Thanks for your time.