Wait until fall arrives to get the best results. If the weather is too hot, you'll have a hard time getting new grass to thrive, and it won't grow at all if it's too cold.
Test your soil to find out if a lack of nutrients is causing your lawn distress. You can rent the equipment to do this yourself, usually from a home and garden store. Many commercial companies also offer soil testing services.
Aerate your lawn before you lay down any new seed. Mow the grass short, to about 1 1/2 inches tall. Use a stiff rake or an aerating tool to break up the soil just enough for new grass to take root.
Spread new seed over dead areas in your lawn. Sprinkle it by hand or use a spreader if the area is very large. Read the manufacturer's instructions to check for required weather conditions and watering requirements.
Water only according to the directions recommended by your seed manufacturer. Once the new grass is established, water only when the weather is above 90 degrees Fahrenheit for more than three days in a row without rain, or when it hasn't rained in over one week. If the grass begins to look dry, it is also a sign that your lawn needs watered. Watering too much is just as damaging to grass as not watering enough.
Fertilize your lawn once or twice a year. This should be done in the spring and/or fall, because fertilizer will scorch your lawn if applied when the weather is too warm.