Remove the plastic as soon as you see any signs of germination. Keep the planting medium evenly moist, using luke-warm water if possible. Water from below to avoid disturbing the tender seedlings. Some gardeners prefer to place the seeded containers under lights immediately after sowing so that they won't have to worry about moving them when they begin to germinate. There are a variety of plant lights available that do a good job.
Fluorescent lights provide strong, cool, inexpensive light to plants.
It is difficult to provide enough light with incandescent bulbs without overheating the seedlings. As soon as the seeds begin to germinate, keep the lights on 16 hours a day, and gradually decrease the time to 12 hours a day as the seedlings mature. Keep the light fixture about 4-6 inches above the plants, adjusting the height as the plants grow. To insure strong, sturdy plants, keep the light within 12 inches. Seedlings not receiving enough light quickly become weak and leggy.