Open the GarageBand app on your iPhone
Tap the plus icon to create a new project
Choose any instrument, then tap the Tracks view
Tap the loop icon and import the audio file you want to use
Drag the audio into the timeline
Trim the audio to 30 seconds or less
Tap the down arrow, then tap My Songs to save the project
Press and hold the project, then tap Share
Tap Ringtone
Enter a name for the ringtone
Tap Export
Tap Use sound as
Select Standard Ringtone, Text Tone, or assign it to a contact
If needed, go to Settings
Tap Sounds & Haptics
Tap Ringtone and select your new ringtone