Question: how to make a song in music library your ringtone on iphone?
Answer: Step 1: Open and update iTunes or Music.
Step 2: Choose a song. Here's the fun part — pick the song that you'd like to use as your new iPhone ringtone. ... 
Step 3: Add the start and stop times. ... 
Step 4: Create an AAC version. ... 
Step 5: Copy the file and delete the old one.