Create a moldmaking box with an open top and bottom that is large enough to enclose your figurine. You can make the box out of Legos if the figurine is small or out of pieces of cardboard taped together. Make sure that the box is at least 1/4 inch larger than the figurine in all dimensions.
Flatten a piece of modeling clay on a work surface so that you have a 1/2-inch--thick rectangle that is slightly larger than the bottom opening of your box. Place the box onto the modeling clay, creating a bottom. The box does not need to be rectangular or square, but it must be larger than your figurine in all dimensions.
Mix together no more than half of your silicone rubber with its catalyst, using a plastic cup and a stirring tool, such as a wooden dowel or Popsicle stick. Make sure that there are no air bubbles in the mixture.
Pour the mixture into the box until it is half filled.
Press the figurine into the rubber, making sure that it is half covered. Do the same to several wooden craft balls or marbles, making impressions around the figurine. These impressions will help you fit the mold together later.
Let the mold sit for at least 12 hours so that the rubber hardens.
Remove the craft impression balls from the first half of the mold. Mix the rest of the silicone rubber and catalyst.
Pour the silicone rubber mixture into the box, over the figurine, until the box is filled and the figurine is completely covered.
Let the mold harden for 12 hours. Remove the mold halves from the box and pull them apart gently, then take out the figurine. The mold should separate with little trouble, as silicone rubber does not easily stick to itself.
Put the mold together, making sure that the edges and impressions line up on each half. Bind the mold halves with rubber bands.
Carve a small hole in the bottom of the mold using a razor blade. Make sure to carve the hole deep enough to reach the figurine impression cavity on the inside.