Choose the audio file you want to use
Trim it to 30 seconds or less
Convert it to M4R format
On Mac, open GarageBand or use Finder and Music
On Windows, use iTunes or Apple Devices app
Import the audio file into the app
Set the start and end points for the ringtone
Export or save the file as a ringtone
Connect your iPhone to the computer if needed
Sync or transfer the ringtone to your iPhone
On iPhone, open Settings
Tap Sounds & Haptics
Tap Ringtone
Select your custom ringtone