If the iDevice Manager is not installed yet, download the program from here.
Connect the device with an USB cable on your computer.
Start the iDevice Manager and press in menu "Tones".
Use the mouse to drag the MP3, M4A or M4R music file from the computer over the list of existing tones in iDevice Manager (Drag 'n Drop).
Alternatively, the files can be picked from a folder on your PC by clicking on the "Select Files" menu entry.
After that can you see details like title, album and artist below the tones list.
If you want, can you play and cut the sound to set start and end position of the tone from your favorite song position.
By pressing "Apply" starts the tone creation process.
After a few moment, is the new tone transferred to your iPhone and you can choose the ringtone.
If the new tone not visible, close the iPhone settings app and restart it. In very rare cases, the iPhone has to be restarted.