Fondant - Covering a cake
Covering a cake with fondant is an acquired technique. It really takes a bit of experiment and finding what works best for you. I personally prefer Sweetshop Fondant You can find that at the website I have linked or at many craft stores. I like that it is very flexible and resilient. I recommend trying a few brands until you find one you like.
I use a measuring tape to measure all sides of the cake I will be covering. Start at the bottom of one side, go all the way across the top, and down the other side. When measuring odd shapes, be sure to lay the tape across all the grooves so you are sure to have enough.
Some fondant is very flexible. With these, the fondant can stretch and I can roll the fondant to an inch less than the size of the cake. Others are not and you should roll either the exact size or about 1/2" more.
Use a small piece of fondant to clean both the rolling pin and the work surface. Even if you have cleaned them both, you may be surprised at the lint you find.
I use a flare technique to avoid folding the fondant on top of itself. Working quickly and starting from the top, I press the fondant onto the cake. I use my other hand to straighten and flare out the rest of the fondant.
Once the cake is covered, use a fondant smoother all around the cake making sure not to trap any air. Use a wheel to cut most of the extra fondant. Then, use a flexible plastic bench scrapper/dough cutter to carefully trim the cake.
This video may help.
I know this looks easy. Sometimes it can be frustrating. Play with the fondant. Learn to recognize how it reacts in your hands. Start small covering an easy cake and before you know it, you will have your own personal technique that works best for you.