Making a 3D cake at home can be as simple or complex as you like. For my daughter's birthday, she wanted a dinosaur cake, and I didn't think the simple pan cake with a picture or figurines would do. I researched different cake styles, pans, ideas and came up with building my own cake.
Using simple box cake mix, with a few tweaks to make it stand up to a lot of icing and a little imagination, I was able to come up with a real hit for her birthday celebration. It all comes down to cutting basic round cake layers and I make a rectangle pan cake and have found that the small egg shaped pan works very well to add some shaping to the cake for legs, facial features etc. A crumb coat layer is essential in building the cake and using the icing to stick the layers together.
I actually get something to look at for the shape like a toy or picture for inspiration and draw out a sketch of how I am going to use the cut pieces and put them together on my cake board. I then actually put the pieces together and move them around until I get exactly the scale and shape that I want.
When that is done, glue them together with the icing and prepare to frost, which is the most time consuming aspect. Using a basic butter cream frosting recipe, frosting bag and basic tips (star tip and round tip), color gels for coloring and a lot of patience you can bring your shape to life. There are many resources online for inspiration and help.