Before you apply the seed, you will need to prepare your lawn by raking it thoroughly with a light rake. This process allows you to remove dead grass, pull up weak grass and generally help uncover areas of your lawn in which the grass is sparse. You can rake your lawn for this purpose in September, as this month still has enough hot sunlight for the reseeding process. Next, use a heavy rake to rough up the sparse areas of your lawn. Over the summer, the dirt can get dry and compact; the heavy rake will help turn it up again.
Once you have raked your lawn and turned up areas in which the grass is sparse, sprinkle peat moss over the lawn, focusing especially on the areas where there's more dirt than grass. The peat moss will help provide the area with useful nutrients and hold the water you will eventually add.
After preparing the area, you can apply the grass seed where you need it most. Avoid using a grass seed spreader. Spreading the seed by hand allows you to accurately focus on the specific areas of the lawn that need the most attention. Scoop a handful of grass seed in either hand and walk around the yard shaking the seeds on to the grass. Don't be sparse with the application, but don't be too liberal, either. A consistent layer of seed over each patch of dirt is ideal.
Immediately after reseeding the lawn, water it lightly but make sure you hit every area. To Help protect the seed from the wind, lay a sheet of plastic or a tarp over the area and weigh it down with rocks. The tarp will also help hold the moisture around the seeds and prevent it from evaporating quickly. Check under the tarp after roughly a week; when you begin to see the new grass growing, remove the tarp and water as needed.