The only way I know how to do it is by syncing them into iTunes by adding them as individual files. Then make sure your iPhone has sync all ringtones selected. Make sure your ringtones and alert sounds have the correct file extension.
There are videos and help sections on the net. I make my ringtones by editing a song or sound on audacity or sound booth what ever you have. A ringtone is a maximum of 30 seconds 29 to be sure. Save the file as an m4a. Which is mp4 audio file.
Find the file you just made on your computer. I would advise the desktop for now. Backspace the file extension and type in .m4r (this is the extension that tells iTunes to see file as ringtone)