A faster way (for me at least, I have a slow connection and this method only takes a minute if you've done it before) is to use iTunes:
1. Select the part of the song you want to use: right-click it, click Get Info, then Options, then set the start and stop time. You can use decimals to be more exact.
2. With the song selected, click Advanced in your menu bar, and click Create AAC Version. If it shows another format, go to iTunes Preferences, and Import Settings, then choose AAC Encoder in the top box.
3. Drag the new song (which should be listed right under the original if sorted by name) to your desktop, then remove it from your library without deleting the file.
4. Rename the file on the desktop, changing its extension from .m4a to .m4r, then drag it back into iTunes and it should be a ringtone.
Remember to set the start/stop times back to the originals.