Open iTunes.
Open iTunes Preferences. This can be done by clicking "Edit" and selecting "Preferences" on a Windows computer or clicking "iTunes" and selecting "Preferences" on a Macintosh computer.
Under the "General" tab, click the "Import Settings" button. Then click the drop-down menu labeled "Import Using" and select the "MP3 Encoder."
Click "OK" to confirm this selection. Click "OK" to exit the Preferences menu.
Click to highlight the iTunes file in your library that you would like to turn into an MP3 file. Then click "Advanced" and select "Create MP3 Version." Alternatively, right-click the file and select "Create MP3 Version." An MP3 version of the iTunes file will then appear below the original file. You have now successfully turned an iTunes file into an MP3 file.