Steps in Repairing Lawns: Corrects only those areas that need work.
Steps in Lawn Renovation: involves killing off the entire lawn and reseeding the lawn.
Restablishing a Lawn: involves not only the complete removal of the existing lawn, adding soil amendments and re-grading the lawn prior to either installing sod, sprigs, plugs, or reseeding.
The repair option is fixing just those areas that need it, and giving the entire lawn a good dose of tender loving care with some fertilizer and weed controls following label directions— better yet, hire a pro to put you on a regular fertilization and weed control schedule! With professional service following an optimized schedule of treatments and applications, a lawn won't get so bad that you'll have to worry about repairs or renovations. In time, you'll have a lawn that you won't be embarrassed to have people talking about.
Repairing a lawn requires a good match between new seed/sod/plus with what is already growing so it's a nice blend. For example: if you live in the north and have a blend of bluegrass (very common) and you've got a few dead spots that didn't green-up this past spring, you'll want to plant a bluegrass blend. Don't put in tall turf type fescue because you heard it's really great (which it is). They are two different types of grass that shouldn't be mixed.
Now, if the bad areas are small, less than a foot, ignore them. They'll fill by themselves in a month or so. Larger than a foot, then it's time to take some action.
Step One: rake out the dead areas. Work in some peat moss or a little topsoil. Now plant your seed (follow label directions). In the south, put in matching sprigs or plugs. For seeds, you can cover with straw, which is good for areas that might be damaged by heavy rain. Otherwise, just a little peat moss on top will work just fine.
When planting the seeds, don't bury them in the ground. Use just enough top soil to barely cover the seeds (no more than ¼", with less being preferable). After covering, press down on the dry soil with the back of a hoe, or use your shoe and lightly tamp it down, but don't stomp on it.
Options: There are some products available that combine a few steps and make it a little easier. Scotts PatchMaster contains just about everything you need: seed at a predetermined rate, fertilizer, and mulch to help keep the seeds properly moist. This works great if you happen to have the same grass type as the package. If not, then don't use it.
Option 2: if you can find sod matching your grass type, you can quickly repair the damaged areas. Follow all of the directions up to the point of planting seed, except for added topsoil. Only add enough topsoil so that with the sod matches your existing level. Sod takes about 2 weeks to establish. Keep it watered and don't let it dry out during those first 2 weeks.
Step Two: Fertilizer the area with a fertilizer specifically designated as a "Starter Fertilizer."
Step Three: Keep the soil moist. For seeds, only the top surface needs to stay moist, but (and this is important, especially if the weather turns hot) never let the soil dry out completely, particularly in the last half of the first 2 weeks after planting. Once seeds germinate, keep the soil evenly moist and increase the amount of water with each watering, but cut back on the number of times you water. In other words, keep the soil moist at a deeper level (moist— not wet!).
Step Four: In a few weeks things will start to pop. Don't walk or mow the areas until the seeds are about 2" — 3" tall, about 3 weeks after the seeds first germinate. Then you can forget about it and just treat it like the rest of the area. Don't use any weed controls on the new grass for a couple of months until it gets really established and hardened off.