You can purchase mature watercress at a supermarket or farmer’s market. Just soak the base of the stalks in water for a few days to encourage root growth and proceed to plant them in soil as you would from seed. You can also start watercress from seeds, which you can find at a farmer's market, gardening store, or online. Watercress grows well in cool, but sunny spots with partial shade. Planting watercress in the shallow portion of a steady-flowing, freshwater stream or creek is ideal, but you can also create your own pool or bog of water. Ideal planting times are in early spring after the last frost, or in early fall before temperatures drop too low. If you have a steady flowing stream or creek, simply mix in four to six inches (10.1 to 15.2 cm) of organic compost into the top six to eight inches (15.2 to 20.3 cm) of soil. If you don’t have an existing water source, dig a hole that is roughly two feet (61 cm) across and 12 inches (35 cm) deep to create a bog. Line the bottom and sides with a large sheet of heavy plastic pond liner, leaving a four inch (15.2 cm) lip at the top and punch a few holes in the sides for drainage. Fill the lined hole with a mixture of one part garden soil, one part coarse builder's sand, one part compost and a handful of fertilizer. If planting next to a stream, make sure the soil is deeply soaked. If you’ve created a growing site, fill the bog to the brim with water. If you’ve created a growing site, water the area every two to three days to ensure it remains thoroughly soaked or install a water pump to keep fresh water circulating through the bog. Sow the seeds 1/4 inch (6.3 mm) deep and roughly 1/2 inch (12.6 mm) apart, and cover with a thin layer of fine gardening soil. You can also start the watercress indoors using the method above or transplant mature plants. However, as the plants can be delicate, they may prove difficult to transplant. Once the watercress has sprouted, thin the seedlings roughly four to six inches (10.1 to 15.2 cm) apart. If small white flowers appear, trim them back with gardening scissors to encourage new growth. Once the plants have grown roughly five to six inches (12.7 to 15.2 cm) in height use kitchen or gardening scissors to trim the top four inches (10.1 cm) of the plant as needed.  Avoid taking more than a third of any plant when cutting to allow the plants enough foliage to continue growing. Periodic harvesting helps encourage new growth.

Summary:
Start growing from mature watercress or seeds. Choose a location for planting. Prepare the growing site. Create a growing site. Water the growing site. Plant the watercress. Cultivate the watercress. Harvest the watercress.