In a large microwaveable bowl melt the marshmellows like you were making rice crispy treats. becarefull not to walk away because they will melt then expand and could over flow.
WARNING MELLOW WILL BE HOT!!!!!!!!!!! candy burns are the worst.
When your mellow is all gooey you add the powder sugar (if you have a stand mixer add the goo into it before adding sugar, use the dough hook on med speed) mix the powder sugar in 2 cups untill you reach a bread dough like form. This is also a good time to add coloring and flavoring. (if you want multi color simply work untill you have your dough and split into smaller portions to color each one)
When you have your dough mixed you can fold it by hand you 'll need a little powder sugar to keep from sticking.
To decorate simply spread a thin layer of powder sugar on your CLEAN counter top and roll the fondant with a rolling pin. for round cakes create a circle that will you can drape over your cake and it covers the side. For a square cake roll out a log.
If you have any other questions feel free to ask.