Computer download: Download the mp3 to your computer, tablet, or Mac and either transfer the file to your mobile device or download the file straight to your mobile device. You can use file transfer software like Samsung Smart Switch or the Android File Transfer app to drag and drop the mp3 directly into your "ringtones" folder. Then, go to "Settings". Click "Sounds and vibration". Click "Ringtones". Find it and set it!
Mobile Download: Download the mp3 directly to your mobile device. Go to "Settings". Click "Sounds and Vibration". Click "Ringtone". Scroll until you see "Add from phone" and click that. Scroll until you find it (folder tree varies on various devices). Click it! Click "Done".
If these instructions don't work, just Google "How to add a custom mp3 ringtone to (whatever Android device you have) and figure that shit out. :) Enjoy!