Gather the plainest soap you can find (plain glycerine soap is best, but Ivory or another mild, unscented soap works as well), a soap mold, and a double boiler.
Coat your soap mold with vegetable oil.
Heat 1/3 cup water in a double boiler to a simmer.
Crumble 3 to 4 tablespoons of the kind of herb that you prefer into the water. (Lavender has a lovely fragrance)
Take the double boiler off the heat and allow the herbs to steep for 15 minutes.
Return the pan to the heat and add the bar of soap, cut into small pieces. (To make the soap melt quicker you can grate the soap)
When the soap is melted, pour the mixture into the soap mold.
Leave at room temperature until the soap has hardened.
Open the mold and remove the soap!
Herb's Tips and More
Do you want to learn more about different herbs? Here is a list of books that I recommend.