First start with a little shortening. I don't measure, just throw a big spoon full in your kitchenaid mixer.
Fill a microwave safe bowl with marshmallows.
Add a little water. just a couple teaspoons. Then mix water into marshmallows with your hand. Make sure that all of the marshmallows are a bit wet. (not soggy wet)
Microwave the marshmallows for 30 seconds. then mix.
Microwave for another 30 seconds, then mix.
If they are not melted yet, then microwave for another 30 seconds. mix (do this as long as you need to. All microwaves differ)
Once the marshmallows are melted, pour them into your mixer on top of the shortening.
Then add a cup of SIFTED powdered sugar. (if your powdered sugar is not sifted you will have lumps in your fondant and you will not be able to fix it. Make sure you have a big bowl of pre sifted powdered sugar ahead of time))
Mix. keep mixing until the marshmallow mixture looks like icing; nice and soft. (if you want to add color, you can do it now. If you need to do multiple colors you can do it by hand kneading it later.
Then add more powdered sugar. Add a cup at a time until it comes to a thicker, but not too thick consistency of fondant. It will be sticky, but not too sticky.
Pat your hands in powdered sugar to help get all of the fondant off the paddle.
Use a spoon to scrape the fondant out of the bowl.
Make sure your work surface is VERY clean, then sift powdered sugar VERY generously onto your work surface.
Knead in powdered sugar if you need to (by hand). I usually need to because i don't want to add too much in the mixer and have to throw away all my hard work.
Then let your fondant sit for a while. Maybe an hour or so until it is room temp. This is the only time you can let your fondant sit out. Any other time your fondant will dry out.
Then roll it out using more powdered sugar under it. It will stick if there is not enough powdered sugar.
Pick up fondant (i drape fondant over both of my forarms to transport fondant to the cake.)
Then cover cake! Tips for a square/rectangle cake; do the corners first by forming them around the cake, then do the sides.
use a small roller to cut the access fondant from the cake.
If you have extra fondant left over, roll it in a ball and cover with shortening. Then store it in a thick ziplock bag. ( I use freezer ziplock bags) It should stay soft for a week or so but i throw mine away after a few days if i'm not going to use it.