Wipe the outside of each terracotta pot with a dry cloth to remove any dust or loose clay. Do not wet it: a damp pot will make the first coat of paint bubble.
Paint the outside of the pot with chalk paint or the acrylic-baking soda mixture, working from the base upward. The first coat will look thin and uneven. That is fine. Let it dry for 30 minutes.
Apply a second coat in smooth horizontal strokes. The paint should now look matte, solid and slightly grainy, which is exactly the chalky texture you want. Dry for another 30 minutes.
Turn the pot upside down so the drainage hole faces upward. Using the size-0 brush and black acrylic, paint a ghost face on what is now the front of the dome: two oval eyes and a small round mouth, positioned in the upper half of the curved surface.
Drop a battery tea light into the drainage hole. The light will glow through the thin terracotta walls and out through the drainage hole, creating a warm halo effect. Switch the tea light on before dropping it in, as the hole is narrow.