To make a poultice take fresh herbs or dried ones that have been soaked in freshly boiled water until soft. Mix them with just enough slippery elm powder to make the poultice stick together if necessary. Place it on the affected part and wrap in a clean cloth. To protect sheets and clothes from stains and help keep the warmth of a poultice in you might consider wrapping the applied poultice and limb/body part in plastic wrap. The practice of poulticing topically administers the essential oil so that it can be absorbed through the skin. The heat of a poultice can also help reduce swelling.
A formentation is a strong herbal tea in which a clean cloth is dipped (the cloth can also be filled with herbs, but this would generally be considered a poultice). The cloth is then applied to the affected part.
Slippery Elm will work as the author above suggests, but flour or cornmeal will work just as well. The idea is to use a substance that will help the poultice stay in one place. Herbs should be simmered until soft, not boiled. Allowing the water to reach the boiling point may destroy vital chemical compounds in the herbs that would be beneficial to healing. If the poultice is intended for a wound that is NOT open I will generally add a dash of alcohol to the herb/cornmeal mix to help coax alcohol soluble compounds out of the herbs.
Heat in a poultice is critical to keep the pores of the skin open to allow for greater penetration of the medicinal compounds. Even though I always start with a hot poultice I typically apply a heating pad over top of it to maintain the heat level constantly. If the poultice is going on an open wound I typically put a healthy amount of minced garlic into the mix. Garlic is both antiseptic and anti-fungal. It does sting a bit though,