Start in one spot and work your away around the plastic. If they're old, they should come off very easily. Mine were practically disintegrating.
The set of earpads comes with the earpads, driver covers, and a plastic piece of some sort that is only needed for the Beyerdynamic headphones. YOU CAN THROW THESE AWAY.
Begin in one spot again, sliding the plastic cuff of the earpad into the groove of the headphones. You may have to stretch them a little bit, but its ok.
I recommend something with a 320kbps bitrate or a .flac file for maximum audiophilia.