Always wear eye protection during science experiments. Never handle sharp objects, open flames, chemicals, or other dangerous objects without adult permission and supervision.
Place the bar of soap in the bowl. The bowl should be microwave safe and big enough to hold about one liter of liquid.
Place the bowl in the microwave and microwave it for two minutes. What's happening?
After the two minutes has ended, do not immediately take the bowl out of the microwave. Wait five minutes first and then remove it, when the foam has cooled.
As an added bonus, you can still use the soap! It makes a great bubble bath.