Fill the cells of a plastic seed-starting 6-pack with seed-starting mix to within 1/2 inch of the top. Firm lightly with your fingertips. Set the flat into a tray of water to allow the soil to moisten from the bottom until the surface of the medium feels wet. Take the flat out of the tray and let it drain for a few hours.
Pour some of the tiny impatiens seeds into a small bowl. Wet the tip of a toothpick and pick up a single seed with it. Rub the seed off of the toothpick onto the surface of the soil. Sow three seeds in each cell, spacing them evenly apart.
Sprinkle just enough medium over the seeds to cover them. Set the flat back into the tray of water until the surface feels wet. Take it out of the tray and let it drain for an hour. Seal the flat in a clear plastic bag to maintain humidity, and poke a few holes in it to allow for adequate air circulation.
Set the seedling flat in a warm, bright spot out of direct sunlight. Impatiens need a 70 to 75 degree Fahrenheit temperature for successful germination. The top of a refrigerator or above a hot water heater are good choices, provided the room is warm. Your impatiens should begin popping up in about 10 to 14 days.
Check the flat every day to make sure that the medium remains evenly moist. Don't allow it to dry out, but it shouldn't be wet or soggy, either. Use a plastic spray bottle to spritz the surface with warm water as needed.
Remove the plastic bag as soon as the seedlings germinate. Move the flat to a sunny spot near a window with southern or southeastern exposure. The temperature should now be dropped to about 60 to 65 degrees Fahrenheit to begin acclimating the plants to their move outside.
Water impatiens less now, but keep the medium just barely moist. Begin feeding a diluted one-fourth strength, all-purpose liquid houseplant fertilizer solution weekly. When seedlings are between 1 to 2 inches tall, choose the most robust plant in each cell. Cut the other two off at the soil line.
Set the seedlings outside in the shade after all danger of frost has passed, 7 to 10 days before planting them in the ground. Protect them from the wind, and bring them back in at night. Repeat this for three days to begin acclimating the seedlings to the outdoors.
Place the plants in filtered sunlight for 2 early-morning hours, then move them into the shade for the rest of the day for 3 to 4 days. Leave them out overnight now.
Plant your impatiens in their forever garden home. Choose a filtered or shaded, well-draining area of your garden or yard. Water them enough to moisten the soil surface evenly. Don't ever allow your impatiens to dry out, but don't make them stand in water, either.