Paint the entire sign with one coat of Gesso. If you don't have Gesso, you can find it in the art department of any craft supply or hobby store. Alternatively, you can use a coat of white acrylic paint. Allow to dry completely.
Paint entire sign with yellow acrylic paint. Let dry and repeat with a second coat.
The stripes do not require a pattern, uneven lines add to the character of this project. After yellow paint has dried, use a medium flat brush to paint on wide red stripes, starting at the bottom of the shirt and working your way upward. Paint a red collar band and sleeve bands on as well. Repeat on the other side. Let dry completely.
Before this step, be certain that the red paint is completely dry or the white will turn pink. Use a large scruffy brush to dab on a white oval in the center of the shirt. Let dry and repeat. Let second coat dry completely.
Use a black craft pen to write "outside playing" on one side and "doing homework" on the other side. Spray both side with acrylic sealer spray and let dry overnight. Hang!
To use: Simply hang on your child's door, or on your front door. Have your child flip the sign to the appropriate side when needed.