Gather all of your leftover soap pieces. Grate each soap piece with a grater until you have enough for one cup. Cutting the soap into tiny pieces works, too.
Fill a saucepan halfway with water and place it on the stove, turning the burner to medium heat. Next, place the glass bowl in the pan, making a homemade double boiler.
Pour the milk into the glass bowl and then add the grated soap. Using water instead of milk also works well.
Stir the soap mixture often to ensure that it does not stick to the bottom. Once the mixture has melted, remove the saucepan from heat.
Spoon the melted soap into the muffin tins. Alternatively,
plastic containers, candy molds or even milk cartons can work as molds for your soap.
Set the soap aside so it will cool and harden to the touch, which usually takes between three and four hours. Placing the molds in the freezer for one hour will help speed up the cooling time.
Remove the soap from the muffin tins and place on a wire rack. This will help the soap to harden completely and can take up to a couple of weeks. Once the soap is cured, or completely dry throughout, you can use the soap.
recipe found on ehow.com