As long as it's not too windy (read: indoors), it's doable. Make an O shape with your mouth, center your tongue in the hole that is made (but not sticking your tongue out of your mouth). At this point the trick is not to blow. Period. At all.
Now comes the hard part to explain. Flex your cheek muscles just slightly, just enough to create a pressure change inside your mouth, and a ring will come out.
Sounds like a few steps, but it will take lots of practice. Good luck!