Hypothetically...
You need to get ahold of a "junk" reg you don't care about, things like Harlow's book, and the proper tools to take things apart/reassemble them.
Then try it. Get yourself a tank with LOW pressure to "practice" with when you put things back together (a used one is a good idea!) and slowly test your work.
Repeat until you're satisfied that what you're doing works.... then its into the pool, and again, satisfy yourself that you're doing things correctly.
Regs aren't difficult or complicated, really. They do demand attention to detail though. Then again, so does replacing the ballcock in your toilet - do it wrong and you flood the bathroom! If that was to happen when you were gone for the day, it would do a LOT of damage to the house, right?
Same deal here really.... take it slow, take notes, draw pictures, and start with something you don't mind trashing if you get in over your head.