Choose an audio file you want to use
Trim it to 30 seconds or less
Convert it to M4R format
Open iTunes or Finder on your computer
Import the audio file into iTunes or Finder
Sync the M4R file to your iPhone
On iPhone, go to Settings
Tap Sounds & Haptics
Tap Ringtone
Select your new ringtone
Use GarageBand on iPhone if you want to create one directly on the device
Export the finished audio as a ringtone from GarageBand
Assign the ringtone in Sounds & Haptics