Cut the posterboard into rectangles measuring 3 x 5 inches. You can measure one of these rectangles, then use it as a pattern for the rest of the cards. You will need to make six of them altogether and they need to be as consistent as possible.
Cut out two matching squares of red, blue and yellow construction paper. These squares should measure 3 x 3 inches, so that when they are placed on top of the white posterboard, there is a clear, inch-long space showing at the top and the bottom of the card.
Glue the squares to the centers of the white pieces of poster board. The sides should be flush with the long sides of the rectangles, but there should be a white space at the top and the bottom. Many Montessori schools use plastic or wooden tablets and the children are discouraged from ever touching the colored parts of the tablets. Your tablets should mimic the appearance of the store-bought ones so that you can help your child practice handling school materials properly.
Use the contact paper to laminate the cards. This will give them some added resiliance as well as making them easy to clean up if more than one child plays with them or you have a sick child at home practicing with colors.
Keep the color cards in an easily accessible location. Even if your child is not enrolled in a Montessori school, you should still use the tablets according to the Montessori guidelines, which means once you have worked with the tablets with your child, he should be able to use them on his own--without help getting them out or putting them away--whenever he wishes.