Question: how to make custom ringtones for iphone on mac?
Answer: Select a track to use as the source. ... 
On the Get Info > Options tab set start and stop times no more than 40 seconds apart, or 30 seconds for text tones.
Under Edit (Win) or iTunes (Mac) > Preferences > General tab > Import Settings choose AAC Encoder > High Quality 128k. ... 
Use File > Convert > Create AAC version.