Here's how I did it...
First, I drilled a couple holes through the roof.
Then I used a sponge brush to apply patio paint. You can also use outdoor acrylic paint.
Next came the messy part. I took some preserved moss, and dabbed it into a whole lot of modge-podge glue. I pressed the moss into the base of the birdhouse, and around the drilled holes. In hind sight, I really should have used the glue gun.
I let it sit for about 30 minutes for the glue to dry. I took some florist wire and thread it through the drilled holes.
Then I inserted a succulent cutting into one of the holes and twisted the wire around the stem to secure the cutting.
All that's left is to hang them on the tree.