I have done this three ways:
1.) Put a piece of wet-or-dry sandpaper on a flat surface, like a piece of
plate glass, and slide the end of the tubing back and forth.
2.) Same as 1.), except use an abrasive paste, like valve lapping compound,
instead of sandpaper.
3.) Hold the edge of the tubing against a belt sander, with a water drip
to keep things wet. If you don't use water, the glass will chip. I have
made scoops out of wine goblets this way.