A guide?, no, I was just thinking how I would do it.
You say you want a cable that's flexible like old game cables, so take one, (or two depending on how long you want it), genuine old controller(s), cut off the cable as close to the ends as possible, (you would then have to splice the two together, if using two cables), then simply solder the usb ends on, possibly taken from your original cable, shouldn't take more than an hour...