Place marshmallows and 2 tablespoons of water in a large microwave-safe bowl. Microwave for 30 seconds on high; stir until mixed well. Continue microwaving in 30 second intervals until melted and smooth.

Place about 5 cups of the powdered sugar on top of the melted marshmallow mixture and gently fold it into the marshmallows.

Grease your hands and your counter generously with shortening. Turn marshmallow mixture onto counter and start kneading it like you would dough. Add more confectioners' sugar as needed and re-grease your hands and the counter as needed.

If the marshmallow fondant is tearing easily, it is too dry; add water (about ½ tablespoon at a time) kneading it until fondant forms a firm, smooth elastic ball that will stretch without tearing.