With iOS 11 and iTunes 12.7, this is a little more tricky that it used to be.  Follow these rough steps:

1. BUY and download the song.  Even though I have Apple music, it did not work unless I bought the song as there was a point that it would not allow modification of a downloaded song that you did not buy.  Something about a protected file.
2. Once downloaded and in iTunes (v12.7 was used), so to File > Convert > Create ACC version
3. This will create a duplicate of the song in iTunes
4. Right click on the song, select Options and then trim the song to about 30 or 40 seconds and save/close
5. Right click on the song, select Show in Windows Explorer
6. In windows explorer, make sure in Options you have “Hide for extensions” turned off
7. In windows explorer, change the extension to .m4r
8. In iTunes, connect your phone, open iTunes and select your phone so you can see the contents of your phone on the left side of iTunes.  You should now see “tones” displayed for your phone
9. In iTunes, on the left select Tones, and you should see any tones already on your phone
10. Open a split screen and drag your new .m4r song into the Tones area of your phone
11. Now sync iTunes with your phone and it should be on your phone
12. Now go to your phone Settings > Sounds > Ringtones and look for the new ringtone to select

Here’s a youtube video to help visualize the process. However I had to tweak the video slightly as it did not work for me exactly as it did on this video