Petunias need light to germinate, so you don't want to cover the seed. Just scatter them on the surface of damp potting soil, press them lightly to be sure they are in contact with the soil and leave them under the lights. The soil needs to be warm (between 70 and 80 deg F) and the seeds need to stay damp. I start them on the heat mat.
Most of my seeds I only bottom water, but the things that need light to germinate, I also sprinkle lightly with water to keep them from drying out while they are exposed to air and have no roots. Once they have sprouted, I quit the sprinkling.
They are kind of slow, that's why I start them in January. By last frost date in April they will be big sturdy 8" or so tall plants and blooming already.