One of the things that I get asked most often about the cakes I make is "Oh my! How on earth do you do that!". So, I figured I'd give some basic instructions for anyone interested. Please keep in mind that I have never done any formal training on this, so there may be some tips or tricks that I have no idea about. This way just works for me.
1. Make sure that the cake you're covering is quite dense. The buttercream and fondant gets quite heavy, so you need a recipe that produces a heavier cake. To make the cake taller than my cake tins, I give them a wipe over with butter and line them with baking paper, like so:
Then whack the cake batter in, bake it and let it cool completely. If there's any warmth in the cake, your icing will melt and slide off, and be generally crappy.
Once your cake is cooked and cooled, you need to cover it with a medium-thick layer of buttercream icing. This is a good recipe, or I often just put some soft butter in a bowl, beat it well and beat in sifted icing sugar until it ends up nice and firm.
If you have a lazy susan, use it. It will make your icing life so much easier. If not, you can place your cake plate on top of an upside down bowl, just try not to knock it over.
Then you need to ice your cake. It's easier to use a long, flat bladed knife or an icing spready thing, than it is a spatula. Put a big dob of icing on the middle of the top of the cake and use the knife to spread it from the centre to the outer edge of the cake until the top of the cake is covered in about half a centemeter of buttercream.
Next step is to ice the sides. I scoop a big blob of icing onto my knife, hold my knife almost against the side of the cake at around a 45 degree angle, and turn the cake around. You may need to do this a few times to cover the whole side. Then run your knife under some hot water, wipe off and use your warm knife to smooth out any little ridges that have formed.
You should end up with something that looks a bit like this.
Then, put the whole thing in the fridge for about an hour. It's much easier to put fondant on firm buttercream.
Now it's time to roll out your fondant. Give it a good knead to make it soft, shape it into a disk and put it onto two sheets of baking paper. Lightly sift a little icing sugar over the top to stop it sticking to your rolling pin.
Start rolling it out. Make sure you're rolling the icing, not pulling it, and with firm pressure start rolling from the centre out. Imagine you're looking at a clock face: start at 12:00 - roll, turn to 2:00 - roll, turn to 4:00 - roll, etc...
...until it's all rolled out into a big circle-ish shape.
Next up you're going to need your cake, your beautifully circle-ish fondant, a sharp knife, a fondant smoother (I have no idea what it's really called, but it's super handy-dandy. Your hand will work too, just not as well), and some icing sugar in a sifter.
You now need to embrace your inner icing-ninja and flip the fondant over onto the cake. It should end up somewhat like this:
Carefully peel your baking paper off and genty use your knife to cut some of your excess fondant off. Roll that up into a ball and wrap in cling wrap for later. Lightly sift a bit of icing sugar over your cake and start using circular motions with your fondant smoothy thinggo to smooth the top of your cake and make the fondant stick to the buttercream.
The sides of the cake are the trickiest bit to smooth. It takes a bit of fiddling but I'm sure you'll work it out. Use your fingers to press the fondant together and onto the sides of the cake, trying not to crease it. You also really want to avoid pulling the fondant, because that will make it tear.
You'll end up with one last crinkle, which you've got to do a lot of working with to get it out, but it will happen! It jsut takes a lot of pulling out and patting back in, but be patient - it'll soon end up like this:
Give the sides and top another once-over with the smoother...
...trim off the last bit of excess, and you're done!
Now feel free to decorate as you see fit. Enjoy!