To make a mustard plaster for a cough, combine dry mustard with flour and warm water to make a paste. While this natural folk remedy has been used for centuries, it can be initially irritating to some people's skin, notes the Wellspring School for Healing Arts.
Mix 1 part dry mustard and 8 to 10 parts flour with enough warm water to make a paste. Add water as needed, and continue mixing until the paste is slightly thinner than pancake batter. Get a three-piece cheesecloth about 12 inches wide and long enough to wrap around a person's chest and back, and spread the mixture on the cloth. Wrap the cloth diagonally across the chest to the lower ribs, around the back, and diagonally across the chest to the opposite shoulder. Wear an old T-shirt, and let the plaster sit for 20 minutes for an adult, and 10 minutes for a child. Remove the cloth, and carefully rinse off the skin. The skin is red after use, explains the Wellspring School for Healing Arts. Repeat once daily for three days.
Using less dry mustard and being extra careful when applying a mustard plaster for the first time can help reduce irritation, according to the Wellspring School for Healing Arts. If you feel a burning sensation, check your skin, and be prepared to wash off the mixture.