First remove all of the actual magnetic tape. (Save the tape and see my other projects on how to crochet the tape into a handle for a VHS cassette clutch!) Wrap the yarn around the spools (you can use the same color or two colors). Be careful not to knot the yarn around the center of the spool; this way the last bit isn't wasted.
Place spools back into the cassette and orient the yarn so it comes out around the screw holes so when put back together the yarn doesn't get crunched. Make sure when placing the two halves back together that the lid goes back on at the same time.