This is a post from the Internet Bonsai Club By Marcus Watts from the UK about mossing your trees to get them ready for a show.
fill it with sphagnum moss – this is fresh moss collected in the autumn, chop it up very fine, dried orchid quality sphagnum works well also but you need to add some water.
Now you add the preferred top layer of moss, pressing it down into the moist under layer. This holds each piece in place as well as providing moisture from underneath. Doing this keeps the moss looking really fresh and the tree benefits too from the added humidity. I wanted a solid moss finish rather than patches here and there but made it from different sizes, colours and types of moss. This gives an interesting surface with several textures.
job done. Finally i added a piece of netting to the top to keep the birds from ripping it all off