How can I create a ringtone from songs I have purchased on itunes?

I known a way about it.
1 Open iTunes.
2 Find the song that you want to make into a ringtone.
3 Listen to the song and find the part of it you want to use. The chorus may be a good place to start.
4 Write down the start and stop times of the clip.
5 Right-click the song and select "Get Info."
6 Click the "Options" tab.
7 Type in the start time of your ringtone in the text box next to "Start Time" in the minutes:seconds (i.e., 2:01) format.
8 Type in the end time of your ringtone in the text box next to "Stop Time." Make sure the ringtone is no more than 40 seconds long.
9 Click "OK."
10 Right-click your song again and select "Convert Selection to AAC." Wait for iTunes to convert your song. It will create a duplicate version.
11 Right-click the ringtone and select "Delete."
12 Click on the "Keep Files" button.
13 Find the file. It's usually in your User folder under "Music > iTunes > iTunes Music" and under the band's name. It will have an extension of m4a.
14 Replace the m4a extension of your ringtone with m4r. You can either double-click slowly to rename your file, or right-click and select "Get Info" on a Mac or "Rename" on a Windows PC.
15 Click "Use .m4r" or the PC equivalent when the system warns you that the change may affect the use of your file.
16 Double-click the ringtone file. ITunes will automatically add it to your ringtones folder in your iTunes Music Library.
17 Connect your iPhone and sync your ringtones.
Hope it can help you.

Here are two options:
http://osxdaily.com/2010/09/04/make-free-iphone-ringtones-in-itunes-10/
http://www.iphoneringtonecreator.biz/itunes-ringtones-creator.htm