Find the favorite music which you want to make as iPhone ringtone
Right click on the song → click Get Info
You will see the detailed information of that song, but that’s not what we need.
Click Options tab, look for Start Time and Stop Time fields.
The maximum length of iPhone ringtone is 30 seconds. You cannot make more than 30 seconds iPhone ringtone.
So, please select the best 30 seconds which will be your iPhone ringtone by setting the start and stop time.
Select Start Time and set the seconds where the ringtone will start.
Select Stop Time and set next 30 seconds from the start time (maximum length: 40 seconds, but only the first 30 seconds will be played during incoming call). If you only want the first 30 seconds of the song, leave the Start Time by default (0:00) and unchecked and set the stop time to 0:30
When you done, click OK.
Right click on the song again, click Create AAC version. iTunes will create the trimmed AAC version of the original song with selected duration between start and stop time.
When done, right click on the AAC version, right below the original song, and click Show in Windows Explorer.
In Windows Explorer, change the filename extension from m4a to be m4r and confirm the changes (see: Change Filename Extension on Windows). Copy the file into a safe and easy-to-find location.
Click File → Add file to Library → on iTunes and select the ringtone file that you just created or you can simply drag and drop the file to iTunes to add the ringtone file into iTunes Ringtones library.
Now connect the device → click on the device on itunes → click the Ringtones tab → select Sync Ringtones option and let All Ringtones option by default.
Click Sync to transfer the custom ringtone to iPhone.
Once finished, Tap Settings → Sounds → Ringtone on your iPhone. You should see your new ringtone in Custom ringtone category (above the Standard ringtone category)
Select the Ringtone to set it as your iPhone Ringtone