Sorry to be a downer, here's my .02 from reading this thread: you're going to be happier and better off purchasing a tune from a professional shop.
You'll need to dedicate many hours reading/researching/adjusting/dynoing your vehicle, otherwise you're going to be frustrated and disappointed in the results. Read this article from HPTuners support to understand how the E78 works:
http://www.hptuners.com/help/VCM-Edi...th_e78_ecm.htm
There are a handful of tuners already mentioned here, both good/bad to use. No need for me to chime in and say which shop is better, I'd recommend you research (sounds like you already have) and call the few you're interested in. Most people are happy to chat on the phone about what they can/can't do with a tune for you specifically. Most of them use a HPTuners RTD through their shop to deliver their tunes to you.
Haven't seen anyone else mention it here yet, but you will also need a Wideband O2 installed and connected to HPTuners MPVI so you can see what your actual AFR is. Without one, you're shooting in the dark when making fueling changes. GM doesn't use Widebands on most vehicles, as an owner and tuner of my 13 Sonic, I can confirm the E78 doesn't (so Trax/Encore don't have one either).
HPTuners supports the E78 in the Sonic/Encore/Trax, worst case your OS isn't mapped which would be a quick update from the support team but I'm going to bet it's already supported now.