Question: how to transfer downloaded ringtones to iphone?
Answer: Open iTunes.
In the "File" tab on top select "Add File to Library" ... 
In the window that came up, select the ringtone you created.
The ringtone you added should appear in the "Tones" (or "Ringtones") folder.
Connect your iPhone to computer.
Select the "Tones" tab and do a Sync.