Plug the card reader into your computer--most card readers utilize USB cables, which are a computer standard. Wait for your computer to recognize the card reader. If you have an iPhone, connect the phone to the computer with its USB charging cable.
Remove the memory storage from the phone and insert it into the appropriate slot of the card reader. If you have an iPhone, the computer should recognize it automatically and it will pop up in your iTunes library underneath the Music section. Otherwise, wait for the computer to recognize the memory inserted into the card reader.
Open the memory inserted into the card reader by double-clicking its drive when it's detected. The memory should automatically be formatted to the phone's specifications, separating the memory into folders such as "Music,"Video" and "Pictures." If you see a folder for music, open it.
Locate the MP3 files in your iTunes and select your desired group of songs. Click and drag these songs into the music folder opened from your phone's memory. Wait for the process of transferring to complete, then safely eject the memory out of the card reader. If you have an iPhone, iTunes has a "Sync" option that will allow you to synchronize your entire iTunes library with your iPhone.
Place the phone's memory back in the phone and turn it on. The phone should recognize the MP3 files, allowing you to listen to your music from your phone at any time.