I've learned bit by bit. I can still remember the first time I was brave enough to take a hose off a regulator where it was leaking, and see if I could put it back together and have it not leak. Somewhere in the back of my brain, I was sure I was supposed to take the whole thing to the dive shop and have them check it afterwards.
Over time, I've learned to service inflators, dry suit inflate and dump valves, replace wing bladders, find and solve leaks in dry suits, replace wrist seals, and do some second stage adjustments. I have an IP gauge and o-ring picks and a bunch of o-rings, most of which seem to belong on gear I have never owned. I have a transfill whip.
I've acquired useful reference books, like the Airspeed Press books.
But I still don't do my own regs. I wish I had the money to sell all the regs I own, buy HOG regs, and go take their class, but I don't. And unless we had all the same sort, I really wouldn't get an opportunity to do enough service to get good at it, I don't think.