Write down the equilibrium equation for the compound you have chosen. In this case, you will obtain "Ca(OH)2 -> Ca^2+ + 2 OH-."
Use the equation above to determine the equilibrium constant (Ksp) of CaOH2. The Ksp is the product of the ion concentrations of each of the products. If a product has a constant in front of it, as in our case, then that product's ion concentration is raised to the constant's power. In this case, you will obtain "Ksp = [Ca2+][OH-]^2."
Find the molar solubility of each of the components at the temperature you are looking for. Various online resources or textbooks can be used for this purpose. In this particular example, take [OH-] = 0.04192 and [Ca2+] = 0.02095 at standard temperature and pressure.
Multiply the two values to obtain the Ksp. The final result is [0.04192][0.02095 ^ 2], which produces Ksp = 1.8 * 10-5 at this particular temperature.