The very best time to seed a new lawn or repair an old one with cool-season grasses such as bluegrass, fescue or ryegrass is between Aug. 15 and Sept. 15. Nights are cooler, and aid to seed germination, there is much less chance of competition from sprouting weeds and the grass has until next summer to become established before having to cope with hot, dry summer weather.

It can be successful as a do-it-yourself project if a few rules are followed closely. Incidentally, it is almost as easy to do it right as wrong.

When selecting seed, choose a blend rather than just one type. Research has shown that the mixture of several grasses produces a better lawn. One of the best mixtures would consist of three different kinds of bluegrass.

If there hasn't been a good rain lately and the soil is dry, give it a good soaking or wait until there is a good rain.

Don't try to improve an existing lawn by adding new topsoil to it. There is no advantage to doing so, unless you want to change the grade or fill in low spots.

Don't waste time and good grass seed by throwing them down on smooth, hard ground. When the seeds germinate, they must have constant moisture to avoid drying out. Rake or scratch up the soil to provide cracks or crevices where it will be easier for the new seedlings to stay moist.

Don't roll your lawn after putting down seed. It can do more harm than good by compacting the soil and interfering with moisture penetration.

A light sprinkling of pest moss or straw over the seeds will help to keep them from drying out. It need not be removed later on.

Light is essential for the germination of bluegrass seed. If planted more than 1/3- to 1/4-inch deep, they may not germinate. Research has shown that if planted even 1/3 inch deep, some may germinate but only the more vigorous seedlings survive.

Don't seed too heavily. Seeds sown too thickly are more likely to be destroyed by diseases. If they survive, they are more likely to mature slowly.

Light watering of the new grass is essential until it has developed a root system. The tiny plants will perish without continuous moisture.

When the new grass is about two inches tall, mow it. If tree leaves have started to fall, keep them raked off so they won't another the grass.

After the grass has been mowed two or three times, fertilize it. This will keep it green longer into the winter and get it off to a quick start next spring.

If you have bare spots in the lawn, spots with nothing growing in them, not even weeds, there must be something wrong with the soil. A female dog can cause bare spots. So can spilled gasoline and oil, or rock salt that was used to de-ice the sidewalk. A large piece of rock just below the soil surface could cause a bare spot.

If weeds won't grow in the soil, it isn't likely that grass will either. Scoop out the soil to a depth of five or six inches and replace it with good top soil. The bare spots can then be sodded or seeded.