I started seed last year on top of my freezer. Had pretty good success. After they sprouted, put them under lights for 12/12. I didn't use any bag. You could use the bag until they sprouted, but once they are up --take the bag away. Put plants a couple of inches away from lights.

Just press the seeds into the growing media but do not cover with more growing media. Keep lights on 24/7 until germination is complete. I do this both for light requirement for germination, and to retain consistent temperature. If you seal flat in plastic bag, the temperatures inside the bag will heat up slightly if you keep them about 1 foot below fluorsecent lights. If kept closer, the temperatures inside the bag will become too high. Target 73F.

After seeds germinate, open bag for 24 hours, but leave flat in bag. Then remove bag. Adjust light so they now are 2-3" above the canopy of the impatiens at all times (which means raising the lights as the plants grow).

Instead of a plastic bag, you can use a plastic dome which covers the flat. This is what I use.

Water from below, and do not let the soil go dry, but do not keep the soil consistently saturated/soaked.