Mix flour and mustard powder in a mixing bowl.
Add a small amount of water to the dry ingredients and mix them together.
Continue adding water until you reach a pancake batter consistency.
Place a 100% cotton kitchen towel on your countertop and pour the mixture onto the towel.
Fold the towel in from all sides to cover the mustard mixture.
Wrap the mustard plaster towel with a separate cloth.
Cover the area of the chest you want to treat with a liberal amount of olive oil.
Place the mustard plaster onto the chest.
Place a heat source such as a rice pad on top of the mustard plaster.
Leave on the body and let the mustard plaster work for no more than 20 minutes. Caution: Check underneath the plaster every two minutes to check the level of redness and remove it if there is potential for blistering.