Start with two wire box wreath frames (10″ to 16″D work best). Buy a few at your crafts store or larger quantities online. Soak some green moss (available at home/garden stores) in water. Squeeze out excess water (being wet helps hold to the shape of the frame). Line each wire frame a thin layer of damp green moss, pressing down the center all the way around.
Update 2019: I have found that it’s easier to lay the moss over the top of the soil instead of lining the frame and flipping. Quickly flip the empty moss lined wreath frame on top of the soil ring, Squeeze it together and tuck in the moss on the sides. Fasten together by tightly wrapping with three pieces of heavy gauge wire (2-10″ and 1-14″ long). After fastening the 2 short pieces, flip it over so that the 2 twisted ends are on the front of the wreath (soon to be hidden by growing succulents). Wrap around the longest wire, bending the excess unto a loop for hanging. Twist wires and bend and stick the points into the wreath (this hides the sharp wires and secures the hanger in place).