Assuming you have the ,p3 file you want to use, I use
Ringtone Maker if I'm doing it on the phone or
Audacity if I'm doing it on a Windows PC. (If someone here requests a ringtone, I'll usually use Audacity, because I usually use my desktop for this site, but if I have the ringtone on the phone and I want to change it, I'll use Ringtone Maker.)
If the mp3 is already what you want - the right level, the right length, etc. - just move it to your Ringtones folder.