Carefully slice off the bottom just enough so it doesn't roll around. You don't need to cut off much from the bottom to stabilize it. (If you accidentally cut it crooked don't worry. It will sit on a slight angle as creative flare.)
Cut out most of the middle leaving a 1/2 inch or less for the sides of the bowl. Note: use a large knife because you'll be cutting into the tough core of the cabbage. I find it's easier to do a little chunk at a time rather than try to carve it all out at once.
Note: the cabbage you cut out can be used to make coleslaw, and you can serve the coleslaw from the cabbage bowl