1. Open iRingtones
To create a new ringtone for your iPhone, open iRingtones from your Applications folder or click the application icon located in your dock.
iRingtones application icon
2. Drop files into iRingtones drop area
When iRingtones has finished launching, it will present you a dropping area as illustrated below. Using drag and drop, select a track from your iTunes Library and drop it right into the application window.
You can also drop files directly from the Finder. iRingtones supports all files that you can open with Quicktime Player (including Windows Media, DivX and other video file types if Perian and Flip4Mac installed), but cannot currently create ringtones from a DRM'ed songs.
3. Adjusting your ringtone
Once the application has loaded your music file, it will show you a handy timeline you will use to isolate the part of the song that you want to turn into a ringtone. Use the little blue arrows to set the begin and end of your ringtone.
Note that you can use the bottom-right slider to zoom the timeline so it is easier to precisely set the beginning and ending time of your tone.
If your iPhone is configured to sync all ringtones automatically, the freshly created ringtone will be added to your mobile phone the next time you sync it.
You can also ask iTunes to sync your iPhone from iRingtones' File menu or using the keyboard shortcut ⇧⌘S.
4. To sum up ...
Creating ringtones with iRingtones is an easy 3-step operation: