Be sure the outside of the jug is dry. Paint a thin layer of purple and green stripes around the jug. (You may want to water down your paint so that it goes on slightly transparent instead of opaque.) Let dry.
Remove the lid from the milk carton and glue a piece of black felt to the top of the cap. Pipe some glue around the sides of the jug lid and wrap the felt around it. Secure with a piece or yarn or rubber band until it dries completely.
Paint two eyes and a big smile on the milk jug with black paint. Let dry.
Add the whites of the eyes with the handle end of a large paint brush. Let dry.
Use the handle end of a small paint brush to dot a small circle of black onto the whites.
Untie the yarn or remove the rubber band from the milk jug lid. Cut the felt around the lid in a zigzag fashion to create the monster's hair.
Spray the dry milk jug with acrylic sealer spray. Let dry and spray again.
When dry, fill the jug with water and put on the lid (hair).
At night, place the small light behind the milk jug to illuminate your monster!