You can also use an sonnerie mp3 app to cut, mix and merge songs into a single ringtone. This is useful if you have a song that you want to turn into a ringtone, but not necessarily a full song. You can also use this tool to combine two or more ringtones into one, which can be especially handy if you're using an Android phone and want a song that's a bit longer than the usual ringtone.
There are also several free ringtone maker apps on the Google Play Store that allow you to create custom ringtones from MP3 files. You can save the ringtone from these apps to your phone, or you can add it to the list of available ringtones in your device's ringtone folder.
If you're using a stock Android device (like Google Pixels and Samsung devices), you can set a custom ringtone simply by heading to the "My Sounds" folder and selecting the file you want to use as your ringtone. Alternatively, you can also search for a custom ringtone app on the Google Play Store and install it on your device.