Grades are very important to the end product; runoff can cause erosion, and loss of fertilizers, nutrients, or any chemicals you may apply. Water logged lawns are just as bad and drainage needs to be installed before the lawn. Many grasses will not take standing in water and need well draining soils to grow. Poor drainage can lead to fungi and diseases setting in. You may need to fill in some areas or do some creative landscaping around these areas. All areas that are to be incorporated into the mown part of the lawn need to be as debris free and the ground as smooth and even as possible. Pieces of wood will decay and soft areas will often begin to sink later if not taken care of in the beginning.

Depending upon the time of year you ca also get rid of the weed problems that arise later.  If nematodes are a problem in your area you need to fumigate the soil before planting; these are hard to deal with after the fact.  If amendments have been added (especially lime) it needs to be worked into the soil and it takes more than a month for acidic reaction with the soil to occur. The ground needs to be fully pulverized into the smallest of particles and this is a good time to add a starter fertilizer high in potassium into the top 5 to 8 inches of soil.

Now is the time to add the watering system, replace soil and let stand a few days to settle the ground. Fill in all areas and use a large tooth rake to smooth out the lawn as it will appear when finished. Make sure it is slightly draining away from the base of the house and remove any rocks, roots, anything left behind the first time.

The soil now needs to be compressed and firmed into the finished look once the seed are planted and the easiest way is to use a roller designed to evenly distribute the weight over the soil and press it into place.  You can also roll before and after spreading the seed to produce a more level lawn. 

Seed can be sown with the use of a spreader to evenly disburse the seed. Sow in one direction and then a different direction, so that it will be in a random pattern.
Lightly rake the seed to make contact with the soil and repack.
In areas where erosion is likely to occur apply a light mulching material to keep the seed in place and water evenly several times a day until the seedlings develop.