When I talk about decorating cakes with fondant, the most common complaint I hear is that it doesn't taste very good, but marshmallow fondant is a whole other issue. This is a very simple recipe and takes only basic ingredients ANYONE can get at their local grocery store--no matter how small the town is that you live in. It has four ingredients:
1-10.5 oz bag of marshmallows (mini is easiest because they melt faster, but you can use large if that's all you have),
2 lbs of powdered sugar,
1 Tbsp of water
1 tsp vanilla (clear is best if you have it unless you're going to color the fondant dark)
I've tried greasing the bowl with a bit of shortening, but that only seems to help for a few minutes, so I've stopped doing it. Make sure you use a microwave-safe bowl, big enough to mix in--a couple of quarts is minimum in my book. You can also do this on a stove top, if you don't have anything microwave safe, but it's easiest in the microwave.
Dump the bag of marshmallows in the bowl and microwave on high for 1 minute. Stir the marshmallows with a large spoon. Microwave an additional twenty seconds at a time if you need to until everything is melted. Add the vanilla and water and give it a little stir so it's somewhat incorporated, then add a few cups of the powdered sugar. This is not rocket science--my sister adds all of the two lbs except a cup or so. I prefer mine a little less stiff, so I use less sugar. The important thing is to get it to a consistency that you can work with--not quite as stiff as playdough. You may want to sift your powdered sugar before you add it since it can have little clumps in it right out of the bag, which you then have to knead smooth or pick out.
Before all of the powdered sugar is mixed in, I usually scrub my hands well, grease them up with some shortening, and hand knead in the sugar until it comes to a consistency that I like--still just a little sticky (it is a sugar product, after all), but it forms a smooth ball. I generally knead the fondant a little longer on the counter--which has been scrubbed and greased down with shortening as well. Some people prefer to use more powdered sugar on the counter or corn starch instead of shortening, but I find that makes the fondant even stiffer and if I'm going to be cutting designs for the cake it can become too stiff to work with after a while. Below is my finished ball of fondant.
*Just a note: if the fondant is too sticky, you can always add more powdered sugar. If it gets too stiff and starts to crack along the curves as you knead it, you can melt and knead in extra marshmallows to soften it up.
Next, if you're going to color the fondant it's best to use gel colors. The liquid kind will thin your fondant out and make it sticky again. If all you have is the liquid kind, AND you plan to color the whole batch the same color, add it before you add the powdered sugar, but be aware that it'll take more than you think! I've found I use a lot more color gel with fondant than I do with frosting, so consider that when you're choosing colors. I did black once and I added a TON of the coloring, and it never did get fully black, but I decided dark charcoal was close enough and went with it. Below is a picture with the gel coloring streaked on it. Sorry about the fuzzy picture.
And the next picture is with the green nearly worked into the lump of fondant. Sometimes it's fun to just roll it out streaky like this and use it, but I did continue to work the fondant until I got the color to go through evenly and it made great leaves.
A couple more notes: If you're looking for really bright, vivid colors you may want to consider buying fondant pre-made, especially if it's just going to be a decoration. Black, red, and hot pink, and hot green, and other truly vivid colors are hard to achieve at home, and I know Wilton makes some fondant prepackaged with different colors. I've never needed anything that vivid, so I've been really happy with homemade. You do need to plan a little extra time if you're going to color much of the fondant because kneading in the color can take a while depending on the shade you're trying for and how big the lump is.
I recommend making and coloring the fondant a few days before you do the cake since it stays good for a couple of months if wrapped tightly and refrigerated. I wrap mine in plastic wrap, then put it in a ziplock with the other individually wrapped colors.