Holding centering rings in place while you glue them can be trying. A properly cut centering ring (wood) will slide onto the tube with a slight friction fit and should stay by themselves. However, sometimes the ring is too loose to stay in place when you make the fillet.
A neat trick is to mark the position of the centering ring and then wrap several layers of tape at the mark. The tape holds up the centering ring so that you don't have to worry about it slipping when you make the fillet or as it dries!