Clay is one of the most difficult soils in which to plant grass due to clay's density, lack of oxygenation, poor nutrient content and improper pH balance. All of these problems can be overcome, however, by conditioning the soil, adding minerals and fertilizer, and sowing the seed properly.
Remove surface rocks and weeds from the soil, then break it up 4 inches deep with a tiller to allow oxygen to penetrate. Rake through the clay to remove any remaining objects and plants.
Apply a fertilizer that contains nitrogen, potassium and phosphorus with a lawn spreader. Apply lime in the same manner to bring down the pH level.
Sow your grass seed with the lawn spreader. The proper setting for the dispersal of the seed will be included on the bag. Scatter the seed evenly by hand if you are seeding a small area.
Spread 1 to 2 inches of straw on top of the seed to hold moisture on the surface of the soil.
Water your grass seed frequently and lightly. If you water too much at one time, you can cause the clay to turn muddy and become dense again. Use just enough water to keep the straw damp and prevent the clay from drying out.