Scrub your bench with a wire brush to roughen the surface. Place the bench in a shady, moist location.
Gather moss by scooping it from the ground with a square-blade shovel. Also scoop up 1 inch of the soil under the moss. Scrape moss from a rock with a putty knife, scraping as close to the rock as possible. Gather moss from a location similar to the location of your bench -- a spot with the same shade and humidity level.
Fill a bucket half full of soil, and add a small amount of water. Mix the soil and water to make a thick mud. Make it the consistency of a milkshake. The amount of water needed will depend upon the moisture level of the soil. Spread a 1/2-inch-thick layer of mud over the bench with a trowel.
Cut the moss into 6-inch squares. Blend a square of moss with a cup of buttermilk in a blender.
Pour the buttermilk mixture evenly over the mud on the bench. A cup of mixture should cover 2 square feet of surface. Use a wallpaper glue brush to coat any areas where you cannot pour the mixture.
Water the bench every day for three weeks. If the weather is dry, water it twice a day. Keep the moss healthy by keeping it in the shade and moist.