Open the GarageBand app on your iPhone
Tap the plus button to create a new project
Choose any instrument
Tap the Tracks view icon
Tap the loop icon and import the song you want
Drag the song into the timeline
Trim the song to 30 seconds or less
Tap the down arrow in the top-left corner
Tap My Songs
Press and hold the project file
Tap Share
Tap Ringtone
Name the ringtone
Tap Export
Go to Settings
Tap Sounds & Haptics
Tap Ringtone
Select your new ringtone