No matter what caused your lawn to turn spotty or partially bald, fixing the damage is simple. First, buy seed, sod or plugs of the same variety as your current grass (or a close match). Then proceed as follows.
Remove weeds, dead grass, and other detritus from the injured area (see A).
In cases where dog urine, fertilizer burn, or a spilled substance such as gasoline caused the damage, flush the soil with water before you plant. Cultivate the soil to a depth of 6 to 8 inches (15 to 20 cm).
Spread about a 1/2-inch (12-mm) layer of compost on the surface and work it in well.
If you're using grass seed, sprinkle the seed over the area. Then tamp it lightly with the back of a spade or hoe so that the seed just makes contact with the soil.
Top the seeded spot with straw or other mulch, and water lightly. The soil surface should be kept moist, but not wet, until the seedlings are growing strongly. Then revert to your normal watering schedule.
If you're using sod, cut a piece that fits the cultivated area and set it into place. Then tamp the sod firmly to ensure good contact with the soil below. Keep the sod moist to the depth of the grass roots until the blades show active growth.
To replant with plugs, use a bulb planter to make holes 4 to 6 inches (10 to 15 cm) apart in staggered rows. Then set in the plugs. As with sod, keep the site well watered until you see active growth.
Whichever planting method you've used, lay a wooden board across the repaired spot to make sure the new grass is level with the rest of the lawn (see B).