1. Open the Files by Google app.
2. Go to the folder containing the MP3 file.
3. Tap on the MP3 file.
4. Tap on the three dots in the top right corner.
5. Select Use as ringtone.
6. Adjust the start and end times of the ringtone.
7. Tap on the Set button.
iPhone
1. Open the GarageBand app.
2. Tap on the Audio Recorder button.
3. Tap on the File button in the top-left corner.
4. Select Import Audio > Import Files.
5. Go to the folder containing the MP3 file and select it.
6. Tap on the Edit button.
7. Adjust the start and end times of the ringtone.
8. Tap on the My Songs button in the top-left corner.
9. Tap on the Export button in the top-right corner.
10. Select Ringtone and tap on the Export button.
11. Enter a name for the ringtone and tap on the Export button.