Choose an audio file you want to use as the ringtone
Make sure the clip is 30 seconds or less
Open GarageBand on your iPhone
Create a new project and select Audio Recorder, Keyboard, or any instrument
Tap the Tracks view
Tap the Loop icon
Import the audio file from Files, Music, or iCloud Drive
Drag the audio clip into the timeline
Trim the clip to 30 seconds or less
Tap the down arrow or My Songs to save the project
Press and hold the project, then tap Share
Tap Ringtone
Name the ringtone
Tap Export
Tap Use sound as…
Tap Standard Ringtone or assign it to a contact
If needed, go to Settings
Tap Sounds & Haptics
Tap Ringtone
Select your custom ringtone