These cool days of September present the best opportunity for starting a lawn from seed. Don't dally; you want to get the grass seed started right away to take advantage of the last of the season's heat. Starting a lawn from seed is one of the more challenging gardening tasks, mainly because of all the pre-seeding groundwork.
The first step in starting a lawn "from scratch" is to turn organic matter (and lots of it) into the area to be seeded. The more organic matter that is blended with the soil, the thicker and more drought resistant the lawn will be for years to come. Peat moss, capable of holding over twenty times it's weight of water, is the best choice for improving the soil where a lawn is to be planted. It comes in dry, compressed bales so, it's easy to handle and use. In very sandy soils, you'll want to turn one 4 cu. ft. bale into each 100 sq. ft. (10' x 10') area to a depth of about 6". This sounds like a lot of peat moss but it's worth the effort. For a large area it's wise to rent or borrow a rototiller to thoroughly blend the peat moss with the soil.
Once the peat moss is turned in, the area should be raked as smooth as possible, then rolled smooth with a water-filled roller (another device worth renting for this project). After rolling (whew), any high or low spots will be obvious. Continue to rake and roll until the area has a contour you find acceptable. This is the hardest part but take your time to do it right since you'll be looking at the area for a long time.
Once that's done, lightly rough up the surface of the soil with a metal rake. Finally, it's time to broadcast the grass seed.
Consult with the folks at your local garden center to determine the best grass seed blend for you particular soil and light conditions. For late-summer seeding, avoid cheap blends with large amounts of annual ryegrass. Broadcast the seed evenly at the rate specified over the area then roll it once more to press the seed firmly into contact with the soil. If it's a large area, you'll want to cover the seed with a light layer of straw; a small are can be covered with burlap or horticultural fabric. The reason the seed need to be covered is to keep the sun and wind from drying the seed while it germinates.
After all this is accomplished, you can start watering...and watering...and watering. This is the trickiest and most important part of the project. No matter how high the quality of the seed used, it won't germinate unless the area is kept moist CONSTANTLY. It can't be allowed to dry out; NOT ONCE, not even for an hour. IF IT DRIES, THE SEED DIES AND CAN'T RE-START. Premium blends of fescue and bluegrass will take 2 to 3 weeks just to sprout, so be diligent about watering and don't get discouraged. If you use a blend with perennial ryegrass as well as fescue and bluegrass remember, the ryegrass will sprout a week or more earlier. Even after the ryegrass sprouts, keep watering as if nothing has happened to insure the germination of the desirable fescue and bluegrass seeds.
After the young grass is up,, you should apply a controlled release fall/starter type of lawn food to stimulate quick root growth. Look for a starter fertilizer with a high middle number (phosphorus). When the grass grows to about 4", mow off an inch (and no more) to promote even more root growth. In spring, apply another shot of fall/starter fertilizer to help the young grass develop a mature root system before the summer heats up.