The typical ratio for mixing plaster of paris is 2 parts powder to 1 part water. Use this ratio or follow the instructions on the package of your plaster of paris powder
To make one small batch, mix ½ cup plaster of paris powder and ¼ cup water. Follow the mixing instructions on your brand of plaster of paris powder or use this basic mixing guide.
Mix another batch of colored plaster of paris (repeat Steps 3 to 5) if you are making a two-colored or three-colored chalk.
For a two-colored chalk, pour the mixture all the way to the top of each tray section. For a three-colored chalk, pour the mixture two-thirds of the way.
Mix another colored batch of plaster of paris. Pour this over the second layer to complete a three-colored chalk. Allow this layer to set completely.
Plaster of Paris usually takes about twenty minutes or so to set. You can leave it in longer just to make sure. You will know when it has set when the chalk is rock-hard and cool to the touch.