Use a type/brand of paste that dries out rock-hard (I used Squires Kitchen flower paste). That way the cake topper won't sink. If the gum paste you are using is quite soft, add CMC to it.
Make the legs first and let them dry out completely. They need to be firm before you attach the body, otherwise the legs will sink under the weight of the body.
The legs have sticks (wooden skewers) inside. The sticks need to go as deep in the body as possible. Leave the sticks long and trim them to the depth of the cake once the cake topper is completely dry. If the stick reaches the bottom of the cake, the cake topper won’t sink into the cake.
Build the cake topper on a cake dummy. Make tunnels (for the sticks in the legs) in the 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 tunnels. That way it will be easier to take out the cake topper later.
If the cake on which you are planning to put the cake topper on is soft and/or you are worried that the cake topper might rip through the cake during transport, consider making the cake topper on:
a plaque (I would make if from flower paste so that it dries out rock-hard; about 0.5-1 cm thick)
or on a cake dummy (covered with sugarpaste/fondant).
Make the body and the head slightly flattened. That way they won't be as heavy as if they were round.
Let the body dry before you attach the head. It needs to be firm. That way it won't sink under the weight of the head.
Let the head dry before you attach it. It needs to be firm. That way it won't sink.