Once you've cleaned and repaired your lawn, you may need to reseed parts of it that are particularly bare or brown. This can dramatically improve the appearance of your grass, but there are a few simple steps you should follow to ensure that it won't look worse after you plant than it did before.
First try to address the soil conditions that prevented grass from growing in the past. Call your local Cooperative Extension office to find out where you can get a soil test; this will tell you what nutrients your lawn is lacking. Once you've corrected your soil composition, aerate the ground to avoid any problems with soil compaction.
Now you're ready to buy seed and spread it on your lawn. Before choosing a seed, determine which varieties will work best in your region of the country and with the amount of sunlight in your yard. Then roughly estimate the size of the area where you plan to plant, as seed coverage is recommended in pounds per square foot. If you're spreading the seed over a large area, it is best to use a broadcast spreader, but smaller areas can be seeded by hand.
Don't ignore the grass once you've planted it. Water regularly to maintain soil moisture and fertilize with a slow-release, low-nitrogen product. Mow when the grass reaches 3 or 4 inches (7.6 to 10 centimeters) in height, but try not to trim off more than a half-inch (1-centimeter) as doing so could stress the plant.