1. Once you have downloaded the ringtones, open iTunes. Then drag 'n' drop the ringtones into the Library > Ringtones area.
2. Next, connect your iPhone to your computer. Depending on your settings, sync will either happen automatically, or when you hit the "sync" button in the lower right corner of the main iTunes window.
3. Make sure in your settings for ringtones, "sync ringtones" is checked.
4. If sync is not automatic, hit the sync button on the lower right corner of the iTunes window. Ringtones should upload to iPhone!
5. When sync is complete, disconnect the iPhone. Go to "Settings". Then tap "Sounds" and "Ringtone" - and you will be able to select your new ringtones!