Rake the thin spots with a metal lawn rake to "dethatch" the grass. This dethatching removes the thick under bed of grass and aerates the dirt to allow the seeds to sink into the ground.
Sprinkle fresh top soil over the thin areas. Fresh top soil is high in nutrients that fresh grass seed needs to grow. Do not create mounds of dirt just coat the thin spots with fresh soil.
Sprinkle the seed onto the ground over the thin spots and the surrounding area. For small areas, you can sprinkle by hand. For larger areas, use a seed spreader. This will help the new grass blend in with the grass in the surrounding area.
Water the grass according to the instructions on the grass seed packaging. The amount and frequency will depending on the type of grass, the time of year you plant the grass and the temperatures.