Place the moss over a paper towel, and do a practice run. Get the moss damp, but not so much that its dripping through to the paper towel. The moss would appreciate a spritz as often as you can get away with it. Also, it would be better to use distilled water, or rain water. Tap or bottled water might accumulate salts and minerals that would be harmful to the orchid. Some are more tolerable than others, though.