Clay soil is one of the most common soil problems in the United States, and is one of the worst soils to try and grow grass in. As with most plants, grass demands good drainage, and clay soil traps water around grass roots, rotting it and encouraging disease. Roots also have a tough time pushing through the clay to grow well. Amend your clay soil to fix these problems before you attempt to grow grass.
Turn over the soil and break it up with a tiller. Till the soil at least 1 foot down.
Add organic soil amendments to your yard. Spread a 4-to-6 inch layer of an organic amendment like sawdust, bark mulch, peat moss, grass or straw over the entire area.
Add a nitrogen-based soil amendment like ammonium nitrate or sulfate to balance out the carbon in the organic material, so that the soil is well balanced for grass. Spread 6 lbs. of ammonium nitrate or 10 lbs. of ammonium sulfate per 1,000 square feet of the yard.
Till the layers of amendments into the soil. Go over the entire area with the tiller again, mixing the organic and nitrogen amendments into the soil well. Let the ammendments settle for a day or so before planting your grass.
Spread a grass seed that will do well in your location. Spread it at the rate indicated on the packaging.
Rake the seed into the top layer of the soil gently, being careful not to bury it. Spread a thin layer (1 to 2 inches thick) of straw or other light mulch like grass clippings over the grass seed to keep moisture at the surface.
Water once a day in the mornings for the first month or so, then water as much as recommended for your grass variety.