Here’s how to connect a cd / MP3 player to a headphone amp, specially useful for those who are musicians.
First install the 9V battery, then plug the guitar or the smartphone or the mp3 player (depending on what amp it is) into the source jack, and headphone into the headphone jack. Most amps turn on automatically when you plug the smartphone / mp3 player into it.
You can select the buttons to choose between clean or distorted sounds, and for distorted sounds, you can choose between light and heavy distortion. Adjust volume as needed.
And if you want to play along with an MP3 track, you can plug the headphone out from the MP3 player to the AUX in jack of the headphone amp, using the appropriate stereo cables.
Here’s a video that shows how to get a better Audio from Smartphones and MP3 players: