Plan out your animatronic. Whatever your animatronic is going to be, you need to get the look of it down on paper first. Make sure you draw out all angles of the animatronic and include any close up details you may need. The planning process is extremely important in that it can save you time and money if done right.
Construct a "maquette," or a miniature model. Two dimensional images are good to have on hand, but nothing can make you grasp the look of the intended animatronic like a maquette. Use clay to shape and build a to scale model to work out all the details before building the full size version.
Build a full size sculpture. Use clay to build and detail your sculpture. Use a wood frame if needed. This sculpture will be used to hammer out any last details and will be used as the base for the body molding.
Cast the body of the animatronic by creating a mold. Using latex and plaster, cover your sculpture, piece by piece if it's too big, and let it dry. Once dry, split the mold and detach it from the sculpture. You will now have a perfect negative of the sculpture. The mold can now be reassembled and filled with latex. When the latex is thick enough and dry around the edges, the excess is poured out. The latex "skin" is now ready to be removed from the mold.
Build the electronic skeleton. Using a circuit board that is programmable from a computer, build all of the moving parts necessary for the animatronic. Test out the robotic movements before proceeding.
Combine the body and the electronic skeleton to finish your animatronic. Lay the skin over the skeleton. Attach or glue the moving parts of the skeleton to the parts of the skin it should be moving.
Test your Animatronic. Once your animatronic is ready, run it through a battery of tests, especially anything you'll be using it for in the film. If you find anything wrong, go back and fix it, then test again.