How do I cover up small holes in plaster ceiling?
Half a centimeter? In that case, spackle, sand, and paint. They make color changing spackle (pink to white) so you know when it's dry. Apply with a flat edge (e.g. a putty knife). Don't oversand, just enough to knock any rough edges off. And since the paint has an orange peel look, use a small roller for the touch-up.