Spray liquid weed and feed on the lawn. Weed and feed products are available at garden supply stores.
Mow the lawn two weeks after the weed and feed is applied.
Rake the lawn with a double-duty rake to remove any debris or dead grass. Pull up any weeds by hand.
Break up the soil in spots where you plan to reseed. Use a shovel or other tool to dig an inch or two into the soil and overturn it.
Apply about six seeds per square inch in bare spots on the growing lawn.
Cover the seeds with straw or soil to hold them in place.
Water the seeds frequently enough, by hand or with your sprinkler system, to keep the soil moist and prevent the seeds from drying out.