If you're planning to make a drip topping, put your cake into the refrigerator or freezer now if it's not already there. If you have room, the freezer is better. Of course, if your presentation includes buttercream or other frosting on the cake, it needs to be already on the cake before applying the ganache drip. (!) It's better if you chill your cake at least an hour before applying the drip.
Place the chocolate chips in a small, heat-proof bowl.
Place the cream in a small sauce pan and just bring to a simmer. DO NOT BOIL. Bubbles should be just starting to form. You could do this in the microwave too, but microwaves vary substantially in power, and it's easy to to boil the cream by accident and have it bubble over and make a mess. The sauce pan is safer since you can watch it.
Pour the hot cream over the chocolate. Tip the bowl if needed to be sure that the cream covers the chocolate, but don't stir it yet. Cover the bowl with a saucer to trap the heat and WAIT 5 MINUTES.
After five minutes, starting in the center, swirl the chocolate into the cream. At first it will look like chocolate milk, but in less than a minute you'll start seeing the chocolate shine through, and shortly you'll have the chocolate and cream completely mixed into a glossy ganache.
now WAIT at least 15 minutes. The ganache needs to cool before you can use it.
The spoon techniique involves using a tablespoon (not a soup spoon) to place a spoonful of ganache near the edge of the cake and then nudging a bit of it to drool down the side. If the ganache isn't too warm and the cake is cold enough, it will stop halfway down, like magic, and you've got the drip effect.
TEST to see if your ganache drips the way you want. The ganache should still be warm enough that it drips down the side, and the cake should be cold enough that it chills the ganache and stops the drip midway. If the ganache is too cool and won't drip, you can easily warm it in the microwave (in 5-10 second increments) until it's the proper consistency.
If the drip satisfies you, continue around the edge of the cake, getting drips all the way around.
When you're done, the middle of the cake still won't have any ganache on it. Just use the spoon to apply the ganache to the top of the cake, being careful to not create more uncontrolled drips in the process. You can smooth the top out with a swirl of the spoon or an offset icing knife.