The iPhone FAQ website offers step-by-step instructions on how to create a ringtone using the iPhone's voice memo app. First, open the app, or download it from the app store. Record the desired message. Once the memo has been recorded, look for it in the list of memos, highlight it and click on the share button in the lower left corner. Email the memo to an easily accessible email address.
Retrieve the email, and download the attached voice memo to the computer. Rename the file to .m4r, and save it. Import the memo file into iTunes by double-clicking on it. In iTunes, navigate to the Tones menu, and confirm that the voice memo is appearing there. Connect the iPhone to the computer using a USB cable. Click on the iPhone in the iTunes sidebar menu, and select the Tones tab. Confirm that the Sync Tones box is checked, and decide whether to sync all tones or just the voice memo individually.
Start the sync, and the memo will be copied to the phone. On the iPhone, go to the settings menu, select Sounds, then select Ringtones. The converted voice memo should appear in the list of ringtones. Select the voice memo and it can now be saved and used as the new ringtone.