Prepare the garden bed three weeks before the last expected spring frost. Spread 2 inches of compost and 4 cups of 10-10-10 fertilizer over every 100 square feet of mustard bed. Work the compost and fertilizer into the top 6 inches of the garden soil.
Smooth the surface of the bed with a rake. Mark the planting rows with twine or by making a 1-inch-deep furrow for each row, using the corner of a hoe blade. Space the rows 12 inches apart.
Sprinkle the mustard seeds in each furrow, planting approximately six to eight seeds per foot. Cover the seeds with soil so they are planted 1/2 inch to 1 inch deep.
Mist the soil surface with water as necessary so the top 1 to 2 inches of soil remains moist but not soggy. Mustard germinates in one to two weeks if the seed bed is kept moist and the soil temperature remains between 55 and 65 degrees Fahrenheit.
Thin the mustard plants after they sprout and begin producing their second sets of leaves. Pull the extra plants so those remaining are spaced approximately 3 inches apart in the row.