AT@T sony ericsonWhat kind of cell phone?
I use a program called Wavepad to edit the tracks. You can get a free trial here: http://www.nch.com.au/wavepad/masters.html
After the trial runs out, it just says you have to buy it, but everything still works. Just crop out the sections of the song that you want to use in your ringtone. ~15 seconds works well. Then just upload the mp3 to your cell phone via USB or bluetooth.
If you have bluetooth, just upload it using that. If not, http://www.geekman.ca/rants/ericsson-upload.shtmlAT@T sony ericson