A hand reaches out from a mysterious box in the ceiling and holding a light bulb. I love this idea!
Now I'll try to explain how I realize it.
Step 1: Realization of the hand
When the paper was dry, the hand had to be plaster. I covered the whole hand using a brush and a spatula. At this stage I tried to correct the imperfections due to the mesh, holes, depressions, bumps, etc...
When the grout was still not completely dry, I sponged the surface with a damp sponge, this helped to level out well and you avoided having to sand with sandpaper later.
Then I colored the hand with a paint spray.