Moss is often equated with age - the older a tree or yard ornament is, the mossier it might be. Gardeners who want to give a bird bath or other ornament an aged look can speed up the process of growing moss by thinning buttermilk or yogurt with water and adding bit of moss.
Blend the moss, and equal parts buttermilk (or yogurt) and water together in a blender, and spread on the structure with a paintbrush. Keep the area moist until the moss takes hold and begins to spread.
Other recipes call for the addition of a few ounces of potter's clay (available and art and craft supply stores) to help the mixture adhere better to the surface you want to encourage the moss to grow on.