Buy radish seeds to sprout from a local health food store or use those that you have harvested yourself. Avoid using the seeds sold commercially for planting since these are often treated with chemicals against diseases.
Rinse 1/2 cup of seeds well and strain. Soak in a bowl with 2 1/2 to 3 cups of cold water. As a general rule, use four times as much water as the quantity of seeds you are using. Let seeds soak overnight.
Line a cookie sheet with raised edges with paper towels. Drain seeds the next day and spread out in a cookie sheet. Spray with water to moisten the seeds. The paper towels will help retain moisture.
Place cookie sheet in a warm area with a preferable 60 to 80 degree F temperature. Rinse seeds twice a day, returning to the tray and spraying with water later to keep paper towels moist. The sprouts will start to grow within three to five days.
Remove and use after sprouts are at least one to 1 1/2 inches long. A good time to use them is to wait until 3 to 4 inches long.