Preparing a lawn of already established grass is called overseeding. Reasons for overseeding include thickening an existing lawn, adding a different type of grass or repairing worn or damaged spots. Overseeding is not a difficult task, although it may take some time. The most important step in seeding for a successful lawn is choosing the right grass seed for your area. Seeds are categorized into warm-season and cool-season types. Make sure to check with you local lawn and garden center to see which one you need.
Mow your lawn on the lowest setting on your mower. Use a bagger on the mower if you have one. Rake with a metal rake to remove any debris and leftover cuttings.
Place the seed in the slit seeder and begin seeding the lawn. You can rent a slit seeder at the local tool rental store; it will give you much better results than seeding by hand. Spread extra seed by hand onto bare or damaged spots. Follow manufacturer's recommendations for the amount of seed per square foot. If your existing lawn is very thin, use the recommended amount for starting a new lawn.
Cover the lawn with thin coating of compost and peat moss. For thin areas, you can add more seed by hand on top of the mixture.
Use a water-weighted lawn roller about a third full, and roll the entire lawn to make sure the seed is in contact with the soil. The seed will not germinate if it is not in contact with the soil.
Water well to sink the seed even farther into the soil. Don't allow the seed to dry out until after germination. Water twice a day for at least two weeks until you see the new seed start to grow.
Mow when the old grass reaches about 3 inches. After the seed is established, continue with your regular scheduled weed and feed fertilizer.