There can be many causes of a splotchy lawn. Poor soil can be one; incorrect fertilizing or watering can be another. The improper choice of grass for the amount of sun or shade that a lawn receives can also contribute the the problem, as can pests and disease. It is always a good idea to correct the underlying problem before renovating the lawn itself. Filling in a splotchy lawn with new grass is commonly referred to as "overseeding." Overseeding should be done in fall, after the hottest part of the summer but before winter.
Determine the proper grass seed for your soil and for the amount of sun or shade that your lawn receives. Purchase enough seed to cover the area of your lawn according to the seed manufacturer's directions.
Set your lawnmower to a mowing height of 1 inch and mow your lawn. This will open up the lawn and expose the soil in areas where your grass is thin.
Rent or buy a slit-seeder. This machine will partially rake your lawn while scoring the top of the soil and planting new grass seeds at the same time. Run the slit-seeder across the lawn in one direction (say, east and west) with the seeder set to plant half the recommended amount of grass seed, then run the slit-seeder across your lawn in the perpindicular direction (north and south), planting the other half of the recommended seed.
Spread 1 inch of manure over the top of your seeds using a manure spreader.
Water the lawn thoroughly. Put at least 1 inch of water on the lawn with the first watering, then keep the lawn damp but not soggy for the next 28 days. In hot weather or if there is a lot of drying wind, you might want to water more than once per day. New grass should sprout within 7 to 14 days. Set your mower to a cutting height of 3 inches and mow 28 days after planting your seeds.