I microwaved the double cream and chocolate together in a bowl, stirring every 15 seconds until the chocolate was melted. Then I left it to the side, occasionally stirring until I had the right consistency. If you used it right away, it would be far too thin and would run all the way down your cake. While you're waiting for the ganache to set a bit more, make sure to keep your cake in the fridge. A cooler cake is going to make this a lot easier! You can test the ganache on the side of the bowl to see how far it drips down. I like to put my ganache in a piping bag but you can also just use a spoon. I made the drips all along the side of the cake, then pouring the rest of the ganache on the top of the cake. Use an offset spatula to smooth out the top and you're done!