Adjust the crown to fit the wearer's head if required. Adjusting the crown after it is painted may caused the paint to flake off.
Lightly sand the crown with fine sandpaper, leaving just a few spots shiny. This will help the paint adhere.
Give the crown a coat of matte-finish clear acrylic spray. Leave just a few spots shiny. Matte-finish clear acrylic spray is used to protect surfaces. It is available near the spray paint in craft stores.
Stand several feet away and spray a few areas of the crown with a light coat of black spray paint. Concentrate on raised areas such as the points at the top of the crown. Do not cover the whole crown, just a few areas. Tarnish starts in small dots, so spray lightly.
Add light layers of paint in small areas. Layer the paint more heavily on one side to mimic real tarnish. Avoid some of the areas you left shiny so just a bit of shine remains. Paint until you are pleased with the effect.