First, launch GarageBand. The GarageBand project chooser will open, asking you to select the type of project that you will be working on. From the left-hand sidebar, select "iPhone Ringtone," then select "Voice" from the options, and finally, click the Choose button to begin the project.

In the next window, type a save name, and leave the other options as they are. Click the Create button.

You will now be presented with a new GarageBand project window with two tracks. You'll see Male Basic and Female Basic, and because we selected to have a Ringtone project created, a yellow cycle region (outlined in red) will be added at the top.

Click on the "Male Basic" track and in the Real Instrument area to the right of the timeline, select "No Effects. This will remove any weird audio effects added to your ringtone.

To add a song to one of the tracks from your iTunes library, click the View/Hide Media Browser button shaped like a music note in the lower, right-hand corner of the GarageBand window. Once the Media Browser appears, click on the Audio tab, then click iTunes in the list of options. You can search for your song/artist here.

When you find a song that you’d like to use as a ringtone, drag it to one of the empty tracks in the GarageBand project. After a few seconds, the media will be added to the track you dragged the song to.

After the song is added to the track, you can drag from either side of the track by hovering your mouse over the bottom ends of the track and using the Resize pointer tool that appears to crop it to just the right section of audio you would like to use as a ringtone.

Now drag the yellow cycle area to fit your cropped audio track. This is where the loop will end for your ringtone.

Now that your audio track is perfect for incoming calls, go to the Menu Bar and click Share > Send Ringtone to iTunes. After a few seconds, the ringtone will be created and then automatically exported to iTunes.

The last step is getting the ringtones on your iPhone. Connect your device, then open iTunes and select it in the Devices list. Next, click the Ringtones tab and ensure that the Sync Ringtones check box is checked and that the radio button for All Ringtones or Selected Ringtones is chosen. If you choose the Selected Ringtones option, check the ringtones you'd like to select. When you're done, sync your device and the ringtones will be transferred.

You can then go to Settings > Sounds > Ringtone on your device to select the ringtone you just synced. All of your synced ringtones will appear under the "Custom" heading.