I started building computers over 20 years ago. We use to take our C64's and mod the heck out of them, soldering in new cpu's by piggy backing them on the old one and cutting the main pin and other fun stuff. Then I moved on to the Atari 540, 1040 and Mega ST's and I entered the PC world with the Intel 286 CPU. Back then there was no book or internet to find things out, it was trail and error and it was not all the hard really, but then again I was an electric mechanic and enjoyed playing around with electronics. If you know how to use a screw driver then just grab the old computer and start taking things a part and pay attention to how things come apart.
Heck, I just taught my 5 year old how to take apart an old PIII PC and put it back together, in fact I have the same old pc sitting in my garage right now and all 3 of my kids were taught how to take it apart and but back together so that they can fix things themself when they are out on there on. It not rocket science but does require some common sense but we learn by doing and today you have the world of internet to help and many youtube vids that show you step by step.