This all depends on your cell phone. Some of them require special software that could be hard to obtain, and some phones won't allow external sound files to be added. My girlfriend's phone won't let such files be set as ringtones. So, as you can see, doing this can sometimes be difficult. I have a Nokia 6085 and it's very easy to do this for my phone. All I have to do is plug it in; it requires no drivers or special software. It just works right away and lets me do what I want. Because I've never even heard of your phone model, I can't tell you how easy or hard it will be.
As for actually making the ringtones...it's pretty simple. What I do is convert the MP3 to a WAV, then cut out a 20-second portion of the song with Sound Recorder, convert it back to MP3, and transfer it to my phone. Not the best method, but it's easy.