Nokia Lumia 620 - How do I create ringtones using a Ringtone Maker app on my Lumia device?

Start the Ringtone Maker application. Next choose the option to create ringtone from main view and you will be presented your music library, and then choose the song.

You will be then able to select the length and position of the ringtone from selected song. A note is shown in the view to guide you. Once you press save, you will be asked to give name and option to set this ringtone as your default ringtone.