Write down what you know. Since Kf is a constant, or a number that is always the same, it is often provided in a chart or table in chemistry books. If you don't see a Kf value provided in the problem, flip to the back of the book and look for a Kf table in the appendices. You may not need to calculate a value for yourself.
Rewrite the equation. If Kf isn't supplied, you need to solve for it with a revised freezing point depression equation. Divide the freezing point depression by the molal concentration so you have: Kf = delta Tf / cm.
Insert the values for delta Tf and cm. For instance, if you have a solution with a molality of 0.455 which freezes at 3.17 degrees Celsius, then Kf would equal 3.17 divided by 0.455 or 6.96 degrees Celsius.