I've done this several times and it's so simple. It has to be for me to be able to do it.
All you need is:
4 lead Phone wire, and a phone jack. These you can get from Radio Shack
Assuming that your running a single line jack, you have a wire that has four leads. Only 2 have power running throught them. Assuming, also, that your running this from another jack. Even if your running this from the exterior terminal, it's still fairly easy to figure out.
The hardest part here is concealing the new wire, if that's a concern. I've fed wires through walls with the help of a coathanger and a cordless drill.
The origional line runs from the phone terminal on the exterior of your house to the jack. Open the face of the jack and you'll expose four screws and four colored wires. The origional technition may have only connected two wires, that makes it even easier, but even if all four are connected, its still simple. The new phone wire will have four wires entwined in it. Strip about 1/4 inch of wire on each lead, and match the colors, connecting the wires to the screws with the origional phone wire.
Secure the new jack where you want it and feed the new wire to it. The inside of the jack has the colors marked as to what wire goes where.
It's kind of like falling off a log, once you've done it, you pretty much know how it's done.