Sand your backboard. If there is old paint you will need to sand it off, and if there isn't you will need to sand to create a smooth surface for the primer and paint.
Prime the wood. All you need is exterior wood primer. Three coats of primer will work best to protect your backboard from precipitation, extreme cold or extreme heat. Let the primer dry overnight before moving on.
Mark the center of the bottom of the backboard. Have someone hold the rim centered on this point with the bottom flush with the bottom of the backboard. Trace the rim.
Paint the backboard white, except for the area you just outlined for the rim. You may need two or three coats of exterior white paint to do this. Let the backboard dry between coats and after completion before moving on.
Measure 24 inches from the center of the bottom of the backboard--going in both directions from the center. These points will comprise the outside of the rectangle. Use a square and measure 18 inches from these points. These will be the top corners of the rectangle. Draw lines connecting the dots.
Make marks 2 inches towards the center of the bottom of the backboard from the outer rectangle points you made. Measure up 16 inches from these points and make two more marks. These points are the corners of the inside of the rectangle. Draw lines connecting the points.
Using an orange exterior paint, paint the rectangle. Use painter's tape to keep from getting any orange paint on the rest of the backboard. You will want at least two coats to make sure the white isn't showing through.