To add custom ringtones to your iPhone, you can follow these steps:

1. Choose a song: First, choose a song that you want to use as your custom ringtone. Make sure the song is in a compatible format, such as .m4r, and that it is not protected by digital rights management (DRM).
2. Trim the song: Use a music editing software, such as GarageBand or iTunes, to trim the song to the desired length for your ringtone. The ringtone should be no longer than 30 seconds.
3. Convert the song to .m4r: Once you have trimmed the song, save it as an .m4r file. You can do this in iTunes by right-clicking on the song, selecting “Song Info,” clicking on the “Options” tab, and changing the start and stop times to fit your desired ringtone length. Then, right-click on the song again and select “Create AAC version.” Once the AAC version is created, right-click on it and select “Show in Finder” (on Mac) or “Show in Windows Explorer” (on Windows). Change the file extension from .m4a to .m4r.
4. Add the ringtone to iTunes: Open iTunes and drag the .m4r file into the iTunes window.
5. Sync your iPhone: Connect your iPhone to your computer and sync it with iTunes. Make sure that the “Tones” option is selected in the “On My Device” section of the sidebar.
6. Select your ringtone: On your iPhone, go to “Settings” > “Sounds & Haptics” > “Ringtone” and select your custom ringtone from the list.

By following these steps, you can add a custom ringtone to your iPhone and set it as your default ringtone or assign it to a specific contact.