Make a tunnel in a cake dummy with a wooden skewer. Insert it straight all the way down. Push it in and take it out a couple of times to widen the tunnel. That way it will be easier to take out the cake topper later. 

The part of the stick that goes in the cake dummy (or later in the cake) should be quite long. I usually leave it long and trim it to the depth of the cake later (once the cake topper is dry). If the stick reaches the bottom of the cake, the cake topper won’t sink into the cake. 

For more support (or if the stick doesn’t reach the bottom of the cake) you can insert a dowel (f.ex. a plastic straw) under the cake topper.