So what's the trick? You prepare you ganache layer as normal, making sure you get the top level and the edges sharp. Here's what mine looks like when I'm satisfied with the finish.
I roll out the fondant and apply it in the same way as usual. I still panic every time I do this. You'd think I'd be over that by now as every job always works out! Jessica suggests trimming the fondant to about 1cm of the bottom of the cake. This is your first chance to turn ithe cake upside down.
Get a piece of baking paper and put it on the top of the cake. Put a cake board on top of the paper then carefully get your hand under the cake and flip. This should be no problem as you have a perfectly sized board under the cake too, right?
Flip the cake and put it on the bench. Smooth the excess fondant skirt upwards so that it is perpendicular. Then trim it to the height of the bottom of your cake using a one sided razor blade or a scalpel. Flip it back over and continue smoothing the fondant the same way as usual, making sure the sides are straight and the top is level. Now flip the cake again.
Use your straight edged smoother to smooth the fondant on the edges downwards. That is, in the direction of the top (your cake is upside down). At this point check the tiny space between the edge of your cake and the bench top. Do you see a shadow? The round edge of the cake casts a shadow. Continue smoothing the fondant downwards until you can't see a shadow any more. Switch to fondant smoothing plastic squares if you have them to put the final polish on your fondant. When you turn your cake back over it should look like this:
It takes time to get this technique right. But it's worth it. If you try it let me know how you go!