Load a song, drag the start point to the hook you want and set a length up to 30 seconds. Add fades, press Preview to check it, then download your ringtone clip.
Thirty seconds is the classic ringtone length and the limit iPhones enforce for custom tones. Keeping to it means the clip will be accepted by phone tooling without a second trim.
The tool exports a WAV. Android accepts WAV files copied into the Ringtones folder directly. iPhone needs an M4R, so convert the WAV in your usual desktop tool and import it through your phone's sync software.
Usually yes. A ringtone that starts mid-note is jarring, and a short fade in and out makes the loop sound intentional when the phone repeats it. A quarter of a second at each end is plenty.