I learned the same way as Rhino. After doing my cam swap on my 2002, I decided I wanted to tune it myself and bought HPT. Had no idea how to use it when it showed up. Lots of trial and error, and lots of lurking and asking questions around the tuning forums at LS1Tech and HPT. It isn't a short road, and the learning curve can be a bit steep- you just start small, and start tweaking more and more things as your understanding increases. Fast forward a couple of years, and I had a heads/cam/full bolt-on car running an open-loop, speed density (no MAF) tune. Eventually I got to the point that I was doing tuning for friends, etc.- generally for beer and pizza.
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Whipple 2.9L S/C, ARH LT's, Flowmaster AMT, HPTuners