Set up your workspace in a well-ventilated room and cover your work surface with plastic to protect from damage.
Build a molding box to contain the object for which you are making a RTV rubber mold. Roll out a flat piece of modeling clay that is at least 1/2 inch taller and wider than the object when it is laying flat. Make sure that the modeling clay is level.
Add walls to the molding box, using either Legos or pieces of cardboard. Make sure that the walls are at least 1/2 inch taller than the object when it is laying flat.
Fill in any gaps between the walls or floor of the box with modeling clay.
Set the object into the box to make sure that it fits. When the object is suspended in the center of the box, it should have a clearance of at least 1/4 inch on all sides, top and bottom.
Mix together a small amount of RTV rubber with its catalyst, enough to fill half of your molding box. Stir the mixture to blend the catalyst completely and to remove air bubbles.
Pour the RTV rubber into your molding box, filling it halfway. Press your model object into the rubber until it is half covered, suspended in the rubber mixture. Make sure that no part of the object is touching the walls or floor of the box.
Press a few wooden craft balls or marbles into the rubber, around your model object, in the same way. Let the rubber cure for 12 hours, or overnight.
Remove the wooden craft balls from the rubber, revealing the "keying depressions." Mix together another batch of RTV rubber and pour it into the mold, covering your model object completely. Let the rubber cure for another 12 hours.
Take apart your molding box and peel the rubber mold halves apart. Remove the model object from the center cavity, then put the mold halves back together. Wrap rubber bands around the mold to hold it together, and carve a pouring hole into the top of the mold with a razor blade.