Secure the mold to a work surface hot glue.
Brush a thin layer of petroleum jelly to the entire object and surrounding work surface. You only need a micro-thin layer as a release agent. Be careful not to fill in any fine detail.
Fill a bucket halfway with water.
Hold your hand underwater and squeeze a large dollop of silicone caulk into it.
Form the caulk into a flat disk shape under the water's surface. The water will prevent the caulk from sticking to your hands.
Press the silicone disk into the object. It should be no thicker than a quarter-inch.
Add more silicone disks until the object is covered.
Allow the silicone to cure for several hours until it has hardened. Different caulks take different amounts of time; consult the label for your caulk's curing time.
Add a second layer of silicone caulk disks, pressing them into the first layer. Allow the caulk to cure, then add a third layer.
Remove the object from the mold once the silicone has cured. If it cannot simply pull out, cut a slit up the mold's back side with a razor blade until it can peel off of the object.