If you have one of those stereos that was released in that awkward time between CDs and MP3 players, then it might be missing the now-standard auxiliary port for easy playback. Redditor Esplodies cleverly hacked together an auxiliary port to get out of this exact situation.
This trick only works on CD players that don't have an aux port, but it's surprisingly easy to do. Esplodies used a CD-ROM audio cable and a resistor to plug into the rear port of the CD player. Then, it was wired up to an auxiliary port and drilled a small hole in the front of the stereo.
It will only work on a few different models of stereos, but if yours doesn't fit the bill, you can do it with a little soldering or go all out and add Bluetooth instead. Head over to Reddit for the full picture guide.