Put ¼ cup of flour into a saucepan. Add a cup of water into the flour slowly, mixing as you go until the mixture is smooth.
Heat the flour mixture over medium heat. Stir continuously until this mixture thickens.
Set the saucepan to the side to cool.
Separate the flour paste into four amounts of ¼ cup sizes each into four different small containers or jars with lids once the mixture has completely cooled.
Purchase a variety of powdered tempera paint in several colors. Add 3 tablespoons of powdered tempera paint, the color of your choice, to each of the containers. Add 2 tablespoons of water to each color and mix the colors well.
Add ½ teaspoon of liquid starch to each color if you would like a poster paint with an opaque finish. Instead of liquid starch, add ½ teaspoon of liquid detergent if you would like your poster paint to have a glossy finish.
Store in covered containers with the lids closed tightly and label them according to color.