At the end of the day, the broad concepts of tuning these trucks are not
that complicated.
1. You press the accelerator, which demands some amount of fuel ("Throttle Base Injection Quantity")
2. At that TBIQ, it looks up a desired fuel pressure ("Fuel Pressure Base") dependent on rpm.
3. With this fuel pressure and injection quantity, it determines a length of time in microseconds for the injector to open ("Main Injection Pulse")
4. The exact moment that this pulse is injected is determined by the timing table at the given conditions ("Injection Timing Base")
And just like that, fuel squirts in, goes bang, and you smoke the tires off.
Of course, there's WAY more to it than that, especially once you get down into the nitty gritty details (like post / pilot injection, environmental modifiers, shift defueling, limiter tables, etc.). But I wouldn't worry too much about those details at the start.
Those details are definitely the kinds of things that forums are a big help on. Once you have your head around the basics pretty decently and start diving into specifics, you can typically search around for whatever specific thing you're investigating and find some good info. Here, older dieselplace posts, and efilive forum are where I'll typically search.
In a more general sense, the advice given so far is spot on. Make small changes to ONE thing at a time. Then go drive the truck, datalog it, and observe how it sounds, feels, drives, etc. different than before. I have a tune revision spreadsheet that I type notes and thoughts into as soon as I finish a drive, just so I don't forget. Looking back at those has been super helpful on several occasions.
Lastly, before making a single change to the tune, learn how to datalog. Play with different PIDs, learn the charts and graphs, make dashboards, etc. Also, look at certain parameter you may want to change eventually (timing, for example), then look at all your other inputs and work backwards to compare to your tune file. This can really help understand what's going on.
It's a daunting task, no doubt, but you'll catch on quicker than you think if you're smart and methodical about it. It just takes time.
