I don't know that anyone is going to provide you a step by step manual on building your mod here, probably some links though. You'll probably have to take some initiative on that. Look at the write-ups people have done on their own mods. Breaktru has posted a few in this section. You can find a bunch of them in the modding section of the ECF forum as well.
When building any electronics project, the first thing to start with is a circuit schematic. You can use an existing schematic, but I would recommend you draw one yourself. Even if you are only copying, it will help you understand the circuit and nail down the components you want to use. Any drawing program will do.
It's certainly possible to salvage parts from old consumer electronics, but it's not something I'd do myself. You'll need a specific potentiometer that may or may not be easy to find inside an old stereo or something. If you order parts online from big electronics vendors like Mouser, Digikey, Newark, etc., you can usually get stuff cheap enough to make salvaging not worth the effort.