Here is how I make my Marshmallow fondant -it's an exact science (not really)
I take a 13 oz jar of Marshmallow cream and a bag of powdered sugar and combine them in my kitchen aid with the dough hook.
I start with a couple cups of powdered sugar in the bottom to keep it from sticking. Then I pour in the bottle of cream ( a hot knife helps it come out). Then I pour in some more powdered sugar and start beating. It mixes into little balls. You'll think you've done something wrong but you can't mess this up. I mix till it doesn't look sticky anymore just crumbly. Then I cover my hands and a surface with powdered sugar, dump out the crumbles and start kneading them together. If it gets sticky at all, add more powdered sugar. Keep kneading till you get a fondant consistency. Add food coloring and more powdered sugar as needed. When I'm done rolling and placing the fondant, I use a kitchen brush to brush off any extra powdered sugar.