A very interesting article here on how to grow moss which can apply for a nice lawn for your Bonsai :)
"This is helpful when transplanting mosses that have been collected by scraping or raking. After the mosses have been placed on top of this loosened soil, they will be watered deeply and then walked on. This will re-compress the fluffed soil and act as an temporary mortar to hold the moss down. If transplanting scooped moss colonies, scratching can be done to create a slight depression to keep the soils level. All areas under and around the transplants will need to be tamped down by hand or by walking on them after installation."