Create or obtain the ringtone file in M4R format
Make sure the ringtone is 30 seconds or less
Save the file to your computer
Connect your iPhone to your computer
Open Finder on Mac or iTunes on Windows
Select your iPhone
Drag and drop the M4R file into the Tones section or sync it to your device
Wait for the transfer to complete
On iPhone, open Settings
Tap Sounds & Haptics
Tap Ringtone
Select the new ringtone from the list