I'm going to keep this fairly simple. Do Not Try This if you are clueless about wires, electricity, following instructions, and using tools (including power tools). I'm going to assume that you can operate a saber saw, a drill, electric screwdriver, and so forth and hardly ever cut off an important limb or digit. I'm going to further assume that you understand that electricity likes to run inside conductors, hates to be shorted out, and (in higher voltages) would just love to plunge to ground through your delicate and easily electrofried anatomy. The network wiring itself carries only trivial voltages and currents during normal operation, but who knows what Evil lurks in the hearts of walls, especially the walls of older houses? If you're careless or unlucky, you can cut right through household wiring while doing something as innocuous as cutting out a hole for a circuit box in your wall.
Then there are your local laws and codes, which may or may not be happy with rank amateurs installing their own wiring, even low voltage low current wiring.
So, before we begin, permit me to say proceed at your own risk. It isn't going to be my fault if you burn down your house, electrocute yourself, lop off a finger with a saw, or even follow all my instructions perfectly but end up with a network you can't quite get to operate. Tough. If you wish to avoid risk, pay a professional.
There. Now we can proceed.
If you are still reading, I presume that you're undaunted, ready to DIY, maybe even a bit cocky about your mechanical abilities. Good. It really isn't that hard, nor is it that dangerous, but there are plenty of Complete Idiots out there in the world who can manage to bollix anything up - and then sue you for it, as if their stupidity or plain old bad luck was your fault.
To install your own wiring, you'll need to begin with a trip to Home Depot, or Lowes, or you own local favorite hardware and wiring mecca. A shopping list of tools and parts you will need or find useful:
This isn't as bad as it sounds. A lot of this is the hardware you will install (circuit boxes, plugs and faceplates, wire, patch panel). Some is tools you probably already have. Aside from that, you need the crimper, the stripper, the tester(s), and the puller (a total cost of maybe $120).
Next you need a plan. Central to the plan is your ``wiring closet'' - the place you're going to pull all the circuits back to. This location might be a closet, a shelf, a cupboard, the garage, a big box (like a circuit box) in a wall. Air conditioned space with an electrical receptacle handy is preferred as you'll want to locate the network switch there, maybe a wireless access point, maybe a DSL modem or cable modem or a gateway there, and all of these need power and generate heat.
It is nice to have a cable drop there (and to pull TV cable along with your network wiring if you're house doesn't already have it). It is nice to have BOTH a regular phone line AND a data phone line (if your service box, like mine, is already equipped with a splitter and separate wiring points). You may or may not need some degree of physical security there - at the very least keeping little fingers out (if you have children) is a good idea, or you may just plain need to lock up the equipment.
However, the essential feature of the space for home wiring is that you must be able to pull cables into and out of the space. This means that you need to be able to put a hole into the walls, the ceiling, the floor of this space and access SOME route to whereever you plan to put a plug. In most houses this means that the space will need to be in the top floor (where you can come down from the attic) or the bottom floor (where you can come up from basement or crawl space).
Some houses are nearly impossible to wire without removing a lot of drywall or at least cutting lots of holes in drywall and then patching them closed when you're done. If you live in such a house, you'll have to decide whether or not you want to mess with DIY wiring at all, as it won't be easy or pretty and I'm not going to cover drywall repair methodology herein so you're mostly on your own.
One solution for even this case that I will mention and that isn't too horrible is to use wiring channels and boxes that are glued or screwed onto the drywall from the outside. This leaves one stuck drilling holes through walls here and there to let cables through, but keeps one out of the drywall itself. Maybe if you are a bachelor...
Once this place is located and routes you can pull to all terminal locations established and put down on a plan, you pretty much just do it. Doing it consists of:
Leave a generous hank of cable on both ends. Pull more than one cable at the same time to the same place whereever possible. You can't have too many cables - unused cables are spares, can serve as extra phone lines, and at worst cost a few dollars in ``wasted'' cable and parts if unused. You can, however, have too few cables, and pulling more cables to fix up some room when the rest of the project is done will be a real pain, easily avoided by overwiring now.
Strip an inch or so of jacket from the cable in the room. Using the (usually provided) punch tool, untwist and wire the ends precisely as indicated on the RJ45 plug. You will often see two alternative wirings (each with its color scheme) represented on the plugs. Use either one (A or B) but use the same one on all plugs you wire, on both ends or you'll make a big mess and nothing will work. I generally use A.
The idea is that all the little one pins have to be wired to all the other little one pins, two to two, and so forth. Mixing mixes up your signals and nothing will work.
When all the wires to that location are so wired, snap the RJ45's into the face plates and wire the whole thing into the box (careully pushing surplus wire up into the wall. Do the same thing on the wiring closet end, either terminating the wire (using the same scheme!) in a punchblock or in an ordinary RJ45 in a faceplate.
TEST THIS CIRCUIT now with your tester, and LABEL THIS CIRCUIT at both ends. You need to know that plug 3a (your middle son's room, top plug) is also plug 3a in your main patch panel and tested between those two points.
The hardest part of this whole procedure is probably fishing for the wires in the walls. Some walls are easy and you just drop in one hole and easily find the other down below. Other walls are full of insulation, of fire blocking, of other wires (dread electrical, phone, cable), of plumbing (DON'T mess with plumbing) or you turn out to be between studs four and five up above while the hole you cut in the drywall for the circuit box turns out to be between studes five and six...
Be careful, work systematically, test often.