To get the healing benefits of mustard, you first have to create a mustard poultice:
You will need a piece of cotton, thick paper towel, or old piece of fabric. Mix about 1 teaspoon of mustard powder with 1 tablespoon of flour, then add enough water to the mixture (just enough to make a paste out of the flour and mustard powder). Spread the paste on half of the cloth and fold the cloth over. Alternatively, spread good quality mustard onto the cloth.
For respiratory effects, place the mustard soaked cloth onto the chest (note: the mustard itself is never supposed to touch the skin; there needs to be a layer of cloth between mustard and the skin). Leave this pack on the skin for 15-30 minutes. Remove immediately if it starts to burn the skin (mustard can be spicy!).
For skin effects, place the mustard soaked cloth onto the burn/cut/infected skin. Leave on the skin for 15-30 minutes. Remove immediately if the skin gets too red, sore, or feels like it is 'burning'.