Fill a seed-starting tray with sterile potting soil. Plant each lentil seed about 1 inch deep, and space the lentils 1 inch apart. Put the tray outside in a sunny spot, or set it on a sunny windowsill.
Water the lentils to keep the soil moist. Wait about 10 days for the lentil seedlings to emerge. Lentils germinate when the temperature is around 68 degrees Fahrenheit.
Transfer the lentil seedlings to an 8-inch or larger pot, depending on how many plants you want to keep, as they start to outgrow the tray. Place the pot outside two or three weeks before the last frost date in your area.
Place a trellis beside the pot, if you desire, to give the lentils support as they grow. Provide water for the lentils when the soil in the container feels dry. When the lentil pods start to dry, stop watering.
Pods usually appear three months after lentil germination. Harvest the lentils when the pods on the plant dry.