You will need..
several clumps of moss
1 can of beer (or pot of natural yogurt)
1 teaspoon sugar (half a teaspoon for yogurt)
blender or paint mixer bit on power drill
Soak the moss in water to remove dirt and small stones that may damage your blender or your eyeball. Blend together the moss, beer (or yogurt) and sugar. Do this in small batches as moss can be pretty tangly on blender blades. You should end up with something that looks like a green smoothie.
Apply by painting onto suitable surface, and within a few weeks the moss should reconstitute and grow. A suitable surface is something that is shady and not too dry. Think about where moss grows. You may want to help it along by spray misting your mossy message with water now and then.
An alternative use of this moss medium is squeezing it into balloons (a funnel and an old shampoo bottle works ok) and splatting them against out of reach walls. Some suggest mixing in some non-toxic paint for added eye-catchiness, but that's up to you.
Pictures coming soon.