1. Defeat the Elite Four and become the Champion. This is the most important step, as it unlocks the next part of the storyline.
2. Visit the Radio Tower in Goldenrod City. After becoming Champion, you'll receive a message to visit the Radio Tower.
3. Talk to the Radio Announcer. He will inform you that the Radio Tower has a new feature: you can now collect Radio Cards.
4. Go to the top floor of the Radio Tower. There, you'll find a new character called DJ Mary.
5. Talk to DJ Mary. She will give you the second Radio Card for free!
Important Note: You need to have already obtained the first Radio Card from the Radio Tower before you can receive the second one.
Enjoy your new radio card and all the new music it unlocks!