Prepare a well worked soil by adding compost and other organic matter to it. Although mustard needs a full sun but it can be grown few weeks before the last frost.
Mustard plant loves moisture so make sure the soil is moist. Fish and seaweed fertilizers can be given to the plant every few weeks. This will encourage the overall growth of the plant.
Begin the harvest as soon as the leaves are big enough for your liking. If the plant has reached the height of 6-8 inches then you can cut the whole plant.