The motherboard manual's got to be the best resource for you. If you don't have it you can probably download it from the manufacturer's site.
It is not difficult. You connect a momentary switch (when pushed makes contact, when released breaks contact) to the pins on the motherboard that are marked Power Switch. Every motherboard manual identifies which these pins are. That's all there is to it. Oh, and... Google results for: "connecting PC power switch to the motherboard"
Again, there are really just 2 basic issues: 1) Selection of a appropriate switch. For power on, you want a momentary switch--that can be anything from pushbutton, to rocker, to RF controlled. If you use other types of switches like a toggle, you will run into the problem that most motherboards will interpret a constant signal of >4s to power back down (some bioses let you change that tho). 2) Connecting the switch to the motherboard. Plenty of online shops will sell motherboard headers with leads. You then just need to splice the leads onto your switch terminals. It's been years since I've used a standard "computer" switch. I scavange them from anywhere. This is a picture I just happened to take recently of a diy switch. Then end switch is momentary, the other rocker is a toggle (going to hook up some lighting). I picked up motherboard header terminals I think from performance-pcs, terminated them with some rapid disconnects ends from Home Depot, and voila. navig
