It's easy...

 

STEP 1: Click the "Download" link and either choose to save the file or open it directly in Apple iTunes.

STEP 2: If you didn't open the file directly with iTunes, double click on the file and it will automatically open in iTunes and play.

STEP 3: Connect your Apple iPhone to your computer and select the check box for Sync Ringtones in the Ringtones Tab of your iPhone's contents and then sync your iPhone.

Your Ringtone should now be visible in the Sounds section of your iPhone's Settings. All future ringtones will automatically be added when you sync your iPhone.