I like to think I'm reasonably competant at most things though my wife is probably a better judge on that but she seems happy and I've truly saved an enormous amount of money over the years with DIY.
Mostly, it's a matter of commonsense, reading up, planning and patience, lots of it to start with, and getting the right tools for the job, and the more you do it, the better you can become.
These days I'm happy doing plumbing, electrics, electronics, plastering, brickwork, woodwork, welding, metalwork, I've even got a decent lathe I use to make components though I'd love a milling machine but I'll have a go at most things. I do want to have a crack at foundry work sometime though, casting stuff. I've got a huge pile of old brass taps I want to melt down and make something arty with, a Hepworth or Moore thing, and I even picked up a blacksmiths forge for pennies, gas forge, anvil and tools about ten years ago and plan to have a go at that soon. Roaring fire, heating iron up and bending it into shapes with big hammers, followed by the time honoured tradition of lots of beer afterwards to slake your thirst....a step up from a barbeque. ... Look Helga, I've made a sword...