1. Transfer the downloaded Ringtone file to your computer/Mac.
2. Connect your iPhone to your computer. Plug the USB end of your iPhone's charger into your computer, then plug the other end into your iPhone's charging port.
3. Open iTunes. Its app icon resembles a multicolored musical note on a white background.iTunes may automatically open when you plug in your iPhone.
If prompted to update iTunes, click Download iTunes and wait for the update to complete. You'll have to restart your computer before proceeding.
4. Wait for your iPhone to appear in the sidebar. It may take a few seconds for your iPhone's name (e.g., "Sarah Smith's iPhone") to appear in the left-hand sidebar; when it does, you can continue.
5. Click Tones. This tab is below your iPhone's name in the left-hand sidebar. Doing so will open the "Tones" page, which is where all of your iPhone's current ringtones will appear.
If you don't see this option, first click Name's iPhone (wherein "Name's" is your name) in the left-hand sidebar to display the options here.
You will not see system ringtones (e.g., ringtones that came installed on your iPhone) here.
6. Click a blank space on the "Tones" page. Doing this will ensure that you can paste your ringtone onto the page.
7. Paste in the ringtone file. Press either Ctrl+V (Windows) or ? Command+V (Mac). You should see the ringtone file appear on the "Tones" page.
You can also click and drag the M4R file onto this page and then drop it here.
8. Click the "Device" icon. It's an iPhone-shaped icon in the upper-left side of the iTunes page. This will open your iPhone's page.
9. Scroll down to the "Options" section. This section is at the bottom of the page.
10. Make sure that all of the boxes are unchecked. In order for your ringtones to synchronize with your iPhone, each box here must be unchecked.
11. Click Apply. It's the button on the bottom of the iTunes window. This applies the changes you made to your Syncing options.
12. Click Sync. It's a button in the bottom-right corner of the iTunes window. Doing so will prompt any music that's in iTunes but not on your iPhone to add to your iPhone, including your ringtones.
Once the progress bar at the top of the iTunes window disappears, you can proceed with applying the ringtone.