1. Scrape the marshmallow-sugar mixture out onto the prepared work surface. It will be sticky and lumpy, with lots of sugar that has not been incorporated yet--this is normal.
2.Dust your hands with powdered sugar, and begin to knead the fondant mixture like bread dough, working the sugar into the marshmallow with your hands.
3. Continue to knead the fondant until it smoothes out and loses its stickiness. Add more sugar if necessary, but stop adding sugar once it is smooth--too much sugar will make it stiff and difficult to work with.