Open GarageBand on your iPhone
Tap the plus icon to create a new project
Choose any instrument
Tap the Tracks view icon
Tap the Loop Browser icon
Import or record the sound you want
Trim the sound to 30 seconds or less
Tap the down arrow and select My Songs
Press and hold the project, then tap Share
Tap Ringtone
Name the tone
Tap Export
Open Settings
Tap Sounds & Haptics
Tap Ringtone
Select your new tone