Pour your ganache on top of the cake and gently spread it toward the edges with your spatula. Add more ganache slowly if it's not reaching the edges. Once it reaches the edge it will drip over on its own. You can control the drip by guiding it with your spatula. The more ganache you use, the further down the cake it will drip. Start with a little at a time because you can't really undo it if you use too much at first.