World's easiest silicone mold.

Step 4: Make sure the mold is water tight.

The best way to make sure your mold is watertight is to add a kind of thick-ish layer of silicone to the surface area of your object. Note how the dinosaur is padded by about a 1/2" layer of silicone all around its body. Also, I have left a considerable amount of the dinosaur uncovered, as I am only casting half of this figure.

You want to make sure you can still wiggle your figure out of your mold, without any of it getting caught, otherwise it can be very tricky to extract once your mold has set.