How To Add Ringtones to iPhone (using Mac and Garage Band on iOS 11)
How to make and add custom ringtones to your iPhone using a Mac computer, GarageBand, iTunes, and YouTube.
DISCLAIMER you may have to sync your phone to the computer being used! So if you have any music in your iTunes on your phone that isn’t purchased you may lose it due to syncing!! (It’s also possible that you can get around this but I’m not 100% sure since I personally use Spotify/SoundCloud/YouTube for music and don’t have any music stored on my phone in itunes)
Hope this helped! And if you get stuck but then get unstuck please share with us what you did to solve the problem as it helps out more people! Thanks!