Create or obtain the ringtone file in M4R format
Make sure the ringtone is 30 seconds or less
Save the file to your computer or Files app
Connect your iPhone to your computer if using a computer
Open Finder on Mac or iTunes on Windows or older macOS
Drag the M4R file to your iPhone’s Tones section or sync it
On iPhone, open Settings
Tap Sounds & Haptics
Tap Ringtone
Select the custom ringtone from the list
Tap the back button to save the selection