In advance, slice some bananas into ½" slices and freeze them, in a single layer, on some waxed paper. When they are frozen, put them all in a Zip Loc freezer bag and keep frozen. When it comes time to make your milk shake, place 16 slices of frozen banana into the food processor. Sixteen slices equals the same amount as a medium size (8") banana.
Add a 6 ounce container of fat free banana yogurt and one cup of 2% milk. Run the food processor about 10 seconds. You will end up with about 2½ cups of thick milk shake. This next photo is not a very good one, but I wanted to show you that the final product is thick enough to support a spoon!!
