Place the CD in the CD ROM. A window may pop up that asks what audio device you want to use. One of the options will be to rip music from the CD with Windows Media Player. Highlight that option by clicking on it then press "OK." If you don't want this to happen every time you insert a CD, uncheck the box in the bottom left hand corner that says "Always do selected action." If you have an MP3 file, then you can skip the next steps.
Rip the song. Windows Media Player will open with a list of the songs on the CD. Check the box next to the song that you want to create as a ringback tone. Right click the tab bar at the top. From the Tools menu, select "Options" then click on "Rip Music." Choose where you want to save the file and what format you want to save it as. Click "OK" then click "Start Rip" in the bottom right hand corner. After it's finished, close Windows Media Player.
Open the wave editor and open the music file. Select the portion of the song that you want as a ringtone. The length should be no longer than 30 seconds, and preferably somewhere around 20 seconds.
Copy and paste the selection into a new file. Save the file as a mono, 16 bit, 96kb MP3 file. This insures that the file won't take up too much memory on the phone but the sound quality is still sufficient.
Insert the USB transfer cable into an open port and connect the phone to the other end. Depending on the phone, software will pop up on the screen and allow you to browse the computer. Find the file you saved then browse the contents of the phone. Find the ringtones folder--normally labeled "My Ringtones"--and paste the file into the folder. Disconnect the cable from the phone.