You will need: a turntable, a spatula designed for spreading icing, cardboard rounds, plastic wrap, a frim cake, a good icing recipe and patience.
My first most important tip is always use a very cold cake. In fact I reccommend freezing the layers prior to filling and frosting them.
My second tip is to use a soft (but not too soft) buttercream recipe. My favorite is an Italian Meringue. It is light and fluffy and not too sweet.
After your cake layers have cooled, you will wrap them in plastic wrap and freeze for at least a few hours, overnight is even better.
(Note) You can cut one cake to make two layers if you bake them in the higher sided pans, which you can get at your local craft store. Wilton is a good brand. If you choose to do this, then cut the cakes after they have cooled, but before they are put into the freezer.
You will need cardboard rounds, precut to each cake size that you are planning on using. Two for each cake to slide under the cakes as you work with them and to serve as a base for each finished cake.
Filling the cakes: For a practice cake, I recommend filling the cakes with plain buttercream icing. No fancy mousses just yet! The cold cakes will solidify the buttery mixture quickly so that the layers will not slide around while frosting the cake.
The first layer of frosting that you put on your cake is called a crumb coat. A crumb coat is a thin layer of icing that is used to seal in the crumbs. This makes it much easier to frost without getting those yucky crumbs mixed in.
The crumb coat doesn't have to look good because you will be covering it with a thick layer at the end with a final coat. Once you crumb coat your cake, you need to return it to the fridge or freezer to firm it up a bit.
Now for your final coat: Place the filled and crumb coated cake onto the turntable and plop a large amount of frosting onto the top of the cake. Now spread from the center out with your spatula, being careful not to press too hard. I always use a back and forth motion while turning the table.
You should end up with some icing further out than the edges of the cake. It should be fairly thick, you will be scraping the excess off later. Now work some of that icing down the sides of the cake gently while turning the table. Don't try to make it look great yet. The idea here is to get the buttercream thick on the entire cake and then scrape it down to perfection later.
Now keep adding it to the sides as needed until the whole cake is covered. You are now ready to scrape some of the excess frosting off. Angle the spatula vertically, and using the edge of it, as you turn the table slowly. Some of the icing will form a ledge that is slightly higher than the top of the cake.
Once you get the sides looking pretty good, start working the top edges. Angle the spatula horizontally and start at the opposite side bring the spatula towards you as you turn the table. Be gentle and don't scrape away too much or it will make a divet.
Keep in mind that you will be putting a border around the edges and decorations on top or possibly a another cake on top, so if it is not absolutley perfect you will be able to cover some mistakes or uneveness.
The final step is to heat up the spatula with hot water and then go over the sides one more time and the top once to really make it smooth. You will need a long wide spatula for this.
You can store your cakes in the refrigerator uncovered once they are frosted. Just make sure that there are no dominating odors, because the buttercream will absord some of that.
The same applies to cake decorating!