We start by opening up the old tape player. There are screws in the back of the player that have to be taken out to open it up. Different players will have screws in various places, but this one only had four screws, one in each corner. (If you take out all the obvious screws from your player and it still won't come apart, be sure to check in the battery compartment. Sometimes there might be a screw or two in there as well. Also, note that some times, as in my case, you will have to press Eject to open/release the tape door since, at least on mine, it latched a part of the cassette mechanism and ended up keeping the front half from coming completely loose.)
Once its opened up, you'll need to find and remove any screws that hold the cassette mechanism in. Also, locate the tape play/record head since we'll use the wires coming from it as the input from the MP3 player.