Using a palette knife, apply the buttercream on the sandwiched cakes. Making sure that the corners are neat and smooth all around. Place in the fridge for about 20 minutes and then add another layer of buttercream making sure that it is smooth and neat. Place in the fridge for 20 minutes again to allow the buttercream to firm up making it easier when you are covering with the fondant
Knead the fondant and roll out on a clean dry surface lightly dusted with powdered sugar or corn flour. Cover the individual cakes and using a cake smoother to shape and push out any bumps around the top and the edges. Cut out excess fondant at the base of the cake to use as decorating as you prefer.
I usually just buy white fondant and then colour it with gel or powdered colouring. Try not use the liquid food colouring because it will make the fondant too sticky and a bit more difficult to work with.