What the best way to go about getting a cd changer put in?

My car has an old cd player and I wasn't sure about what steps I should take to get the cd changer. Like for example do i replace the old cd player with a new one and buy the disc changer? or can I leave the existing cd player in and buy a cd changer and get it put in? Im just not sure on how the whole thing works.Any GOOD advice will be greatly apprieciated