If you've downloaded a ringtone and want to use it on your iPhone, there are only a few steps you need to take. First, open the Settings app and tap Sounds. Next, select Ringtone. On this screen, you'll see all of the ringtones that come with your iPhone as well as any custom ones you've added. To set a custom ringtone, simply tap on it and then select Set as Default Ringtone or Assign to Contact. If you want to assign it to a specific contact, make sure you have the Contacts app open and then select the person's name from your list of contacts. After that, just tap their name again and scroll down to Ringtone. From here, you can choose the new ringtone you just added.