Procure live moss from the forest or woods, collecting enough to cover the required areas on your sculpture. Remove as much of the moss's dirt and debris as you can. If the dirt is stubborn, try rubbing the underside of the moss gently with a soft toothbrush to assist you.
Break the moss down in your hands to create tiny pieces. Do not hesitate to make the moss pieces as small as possible; remember that the moss is still alive at this point, as tearing the structure does not inhibit its ability to grow.
Place the moss in a large bowl and pour buttermilk over the top. Use enough buttermilk to make all the bits of moss soggy. In this case, the acidic principles of the buttermilk promote moss growth. Mix the two components together with your hands.
Paint the mixture onto the portions of the sculpture that you want covered. Use the sponge brush to accomplish this task. The mixture should be quite runny at this point, so add more buttermilk if it is still too stodgy for painting.
Cover the sculpture with an opaque plastic bag, such as a black trash bag. This will give the moss a dark place to begin growing on the sculpture.
Allow the sculpture to set in this way for three days. Then remove the bag and place it in your home for a lovely, garden-inspired decoration.