Make sure that you have the tools, supplies, and ingredients that are recommended to be able to successfully decorate a cake with fondant icing. Also, Crisco is a must (it didn't make it into the photo)
Center your cake on the cake base.
If the top of the cakes isn't flat after cooking, you will have to cut a small portion of the top off to make it flat. If you're stacking cakes to make your cake taller, put a layer of icing between the cakes.
Make sure that the bottom of the cakes becomes the top, as the bottom is the part that has conformed to the shape of the pan.
Apply a thin layer of icing to the top and side of the cake. The icing doesn't have to be perfectly smooth as the fondant will cover it, but also with that being said, try to make the layer of icing as even and smooth as you can.
To create coloured fondant, take out only as much fondant as you are going to immediately work with to create one single colour. Make sure the remaining fondant is wrapped in saran wrap as it will dry out quickly. For example, to cover my 6" x 6" cake in the blue sky base colour, I used 1/2 brick of fondant from my marshmallow fondant recipe.
With a small amount of Crisco shortening on your hands work the fondant until it is pliable. If the fondant is really firm, you can put it in the microwave in 6-10 second intervals. Make sure to check for firmness (you should be able to easily push your finger into the fondant) before microwaving again. Be careful, because if the fondant gets hot enough it will start to melt. Also, be careful when handling the fondant when it first comes out of the microwave as it can be very hot.
Add the gel food colour to the fondant. If you want a lighter colour use less food colour, for darker colours use more.
* I prefer gel food colouring over liquid because the colours are more concentrated. Also, when using liquid colouring is can change the consistency of the fondant.
With a small amount of Crisco shortening on your hands knead and work the food colouring into the fondant. I find having a silicone mat or wax paper to knead the fondant on is helpful. Reapply the Crisco shortening to your hands as needed if you find the fondant too sticky.
You can add more food colouring if you want your fondant darker, or you can add a different colour to change the tint. Once the fondant reaches the desired colour and has no more marbling it's ready to work with.
Roll your fondant out onto the roll and cut mat, or a piece of wax paper or parchment paper. Your fondant should be about 1/8" or slightly thicker. If your fondant is too thin it might tear.
I like using the roll and cut mat as it has guidelines on it which makes it easier to judge how big your sheet of fondant is.
When trying to determine the size of your fondant sheet you need to cover a cake, measure across the top of the cake and down each side and add the measurements together.
I usually like to cut the fondant in the general shape of the cake - just to get rid of any excess areas so they don't get in the way.
Center the fondant and drape it over the cake. You can either do this by picking the sheet of fondant up. Or using the mat and flipping it with the fondant to apply it to the cake.
from the top of the cake, start to smooth the sides down. A fondant paddle works wonders to even out any bumps.
I find the easiest way to describe working with marshmallow fondant is like working with playdough. I can roll, cut, and model it into different shapes. When applying fondant on top of fondant I find running a little water over the back to the fondant your adding helps it stick better and stay in place.
* Again as a reminder, make sure to cover any fondant that is not being worked with in saran wrap.
Depending on the design of the cake, to finish off the base I like to add a thin row of piped icing or ribbon of fondant.