Depends on the wire you want to use, I suppose. Guitar Center has some Mogami 4-core that would be good (I've used it). Mogami W2893 (from Redco) is better as it's more flexible. It can be sleeved with Type 1 paracord really easily. Either one you would probably want to remove the outer sheathing and braid the 4 cores.
For a tutorial, I've never opened ATH-M50s; but there are some other posts that should detail it.