An herbal decoction is very similar to a tea infusion. The difference is a decoction is made with dried roots, berries, or barks, which are a lot sturdier than the leaves or flowers used in tea infusions.
Place 1 - 2 tablespoons of the herb in a pot of cold water. Allow the water to nearly boil, then reduce the heat and simmer the herbs for 20 - 40 minutes. Some very sturdy roots can steep for up to an hour. Strain your decoction and use in place of water in your recipe.
Remember, if you are not using the tea in soap or within 3 days of refrigeration, you will need to add a preservative.