Making cables is one of the easiest DIY things you can do in audio, but there's no way to provide step-by-step instructions for recabling everything because of all the different headphones, plugs, and types of wire available. If you're modding a Grado, you're in luck because the construction is pretty consistent across models and the modding scene is vigorous. Chances are pretty good that whatever you want to do has already been done and extensively documented. On the other hand, if you're recabling something less popular with modders, you're probably on your own.
If you're recabling a headphone that uses a socketed cord (like the K702, HD 600, TMA-1, and so on), you don't have to worry about opening up the headphone itself to get at the wires. Otherwise...
If you've never soldered before, it's a good idea to learn how to do that before you go at your headphones. There are a variety of good tutorial videos out there; watch a couple of them. Try building something simple that you'll have a use for, like a headphone extension cord or some interconnects. Use affordable wire and parts for your first project. The cheaper plugs are frequently easier to build with than more expensive ones. If you end up with something that works but it wasn't your final goal, you can usually sell it off for the cost of parts and get your money back for your intended projects. If you end up ruining plugs and wires, you've lost a whole lot less money.
The DIY cable FAQ is available as a reference. Near the bottom is a list of items you'll need before you can safely solder anything cable-related.