Buy or place a musical instrument in your Sim’s home
Click the instrument and choose to practice or play
Raise your Sim’s Music skill by playing regularly
Reach the skill level required to write songs
Click the instrument and select the option to write a song
Wait for the song-writing process to finish
Save the song to your Sim’s inventory when prompted
Use the completed song by clicking the instrument and selecting play or license song if available
Repeat with different instruments to write different types of songs