Milk is a natural moisturizer, so many soap makers want to include milk in their soap. You can use cow's milk, but goat's milk is perhaps more popular. To make things easy, you can use powdered milk. This allows you to easily store the milk when you're not making batches of soap and you won't have to worry about it spoiling.
Melt your soap base. Use a double boiler to melt the base. You can purchase this at a soap making or craft store. Some common examples are glycerin soap and castile soap. The amount that you melt depends on how much soap you want to make.
Mix the powdered milk with water. Use about 1/4 cup of powdered milk for each lb. of your soap base. Put in just enough water to create a thin mixture.
Pour the milk into the melted soap base. Stir thoroughly.
Add color or scent, if desired. A coffee or peach scent makes a nice addition to go along with the milk theme of the soap. You can buy these scents at a soap making or craft store.
Pour the melted soap into your soap mold. Spray with a vegetable spray before you do this so that you can easily remove the soap.
Allow the soap to set. It's best to let it sit overnight. After it's dry, you can simply remove it from the mold and wrap if you're going to sell it or give it as a gift.