You probably haven't laid any cornerstones lately, but this weekend is likely to be the exception. Today or tomorrow, culminating a month of hard and sometimes confusing work on the lawn, you'll be laying a new cornerstone in the landscape: a spectacular lawn that will beautify your home for the next decade or more. Today, run out and photograph the lawn so you have "before and after" evidence of the miracle that's about to happen in front of your house.

Two lawn programs require explanations: the 60-40 lawn where the weeds have been killed and some good grass survives, and the "disaster lawn" where everything has been killed over the past two weeks. First, let us go through the seeding program for the 60-40 lawn.

Begin by setting up the sprinkler and soaking the lawn thoroughly; a 60- to 90-minute watering is a good rule to follow. Of course, if it rained last night or this morning, don't water.

Seed will be applied over the existing lawn, so the practice is called "overseeding." You could overseed the lawn by hand, but the results will reflect it. For best results, use a rotary Cyclone or Spyker spreader; if you don't have one, borrow from a neighbor or rent one at the garden shop.

Spreader settings for overseeding are as follows: Kentucky bluegrass, setting number 3; fine fescue, number 6; tall fescue and perennial rye, number 6 1/2; rough bluegrass, number 3 1/4. Apply seed once as you walk over the lawn, making sure you overlap the edges slightly as you reverse directions. Seed the border of the lawn first, then the inner area.

After seeding, water the lawn again to wash seed from the tops of the grass down to soil level. Seed must come in contact with soil and be covered lightly with soil if germination is to occur.

On the following day, examine the lawn closely for bare spots. In these spots, seed must be covered if grass is to fill in the area. Where possible, use a hand trowel or cultivator to scratch the soil gently in the bare spot to barely cover the seed. In most cases this is all you need do, but there will be other situations where even gentle hand raking is out of the question because raking would unearth healthy grass.

You solve the problem by covering the seed in the bare spots with filtered soil. Use a 50-50 mixture of sharp sand and peat humus (Michigan peat) to cover the seed. Mix equal amounts in a wheelbarrow, put some in a bucket, then stand over the bare spot and let the soil flow gently through your fingers. You should barely cover the seed with soil, then move on to the next bare spot and similarly cover the seed.

Some lawns with many bare spots may need a dozen or more bags of sand and peat humus for coverage. For economy, leafmold or Compro may be substituted for peat humus, but not topsoil; the quality of topsoil varies so much that it shouldn't be used for covering seed.

Having covered seed in the bare spots, you begin the twice-a-day ritual of spray-misting the lawn, in the morning before 10 and anytime in the evening. Open the nozzle just barely so you have a misting spray, then spray-mist the lawn for four minutes per 1,000 square feet. Keep the nozzle moving to moisten the soil without creating puddles. Do not use sprinklers.

Plan on daily misting for two weeks for tall fescue, fine fescue and perennial rye, but four weeks for bluegrass, which takes all 2l days to sprout. During this time, do not cut the old grass. Thanks to the watering and the fertilizer that will follow next week, the "old" grass is going to get healthy in a hurry and start growing quickly.

Moving on to the "disaster lawn," an altogether different care program should be set in motion this weekend.

First, set up your sprinkler and soak the lawn thoroughly; if it rained overnight, you won't have to soak. A moist soil is an insurance policy for timely germination of the seed.

Here are the appropriate settings for the rotary Cyclone or Spyker spreader: Kentucky bluegrass, number 3 1/2; fine fescue, number 6 1/2; tall fescue, number 8 to 8 1/4; perennial rye, number 7 3/4; rough bluegrass, number 4. Seed only once at 90-degree angles.

Next, get your steel rake (like that used in the vegetable garden) and start raking the seed into the soil. The best method is to rake to and fro, keeping the tines of the rake in contact with the soil at all times. Gently rake so the seed is covered by only a quarter-inch of soil. All seed must be covered this way, so don't rush the job. Wear gloves so you don't blister your hands. Raking is time consuming, but you can't build a lawn without it.

If your lawn slopes to any degree, follow the raking with an application of salt hay over the soil to prevent soil erosion and the loss of seed in the event of a downpour. For this, there is no substitute for salt hay, which costs about $10 a bale. It is pulled apart and applied lightly over the lawn surface. A bale will cover about 1,200 square feet. Again, salt hay is needed only on sloping "disaster" lawns.

Next, start spray-misting the lawn, over the salt hay or the bare soil where seed has been raked in. Plan on misting for only four minutes for each 1,000 square feet; spray in the morning and evening. If it rains, suspend watering for the day.

Daily watering is a bothersome ritual, but it is a necessary phase of your fall lawn program. If you don't water every day, seed germination will be compromised and the lawn will reflect it. Considering how much is at stake right now, the daily sprinklings are a minuscule price to pay for a superior lawn.

How long will you water? At the very least, plan on at least two weeks if you are seeding with tall fescue, fine fescue or perennial rye. These seeds sprout in from 6 to 14 days, so a two-week watering regimen is beneficial. If you water daily for three weeks, reluctant seed will sprout for you. Bluegrass takes three weeks to germinate, so a four-week watering plan is a minimum.

In the first days after seeding (the 60-40 or the disaster lawn), plan on liming almost immediately if you did not lime in the spring. Ideally, take soil samples at a depth of four inches from several parts of the lawn, then have them analyzed for pH by a garden shop; the manager will suggest the amount of lime to be applied to every 1,000 square feet of lawn. Use a drop spreader wide open to apply dolomitic, pulverized or ground limestone. As many as six or seven limings may be made up to late November if the soil test warrants it.

Other lawn hints worth noting: Your current seeding effort is not a do-or-die crisis. No matter which lawn program you are following, you will be able to make a second and third overseeding in the next few weeks to thicken up the lawn.

You probably will have seed left after this weekend's work, so put it aside until you see the results of your first seeding. Perennial rye will be popping up during the Labor Day weekend, so you can easily make a second overseeding the following week. Tall fescue will be up by the Wednesday after Labor Day, and fine fescue by the following weekend. A warning to readers with sunny lawns. If you neglected to treat the lawn this summer for grubs, do so now; otherwise they will destroy the grass plants before they sprout. Apply a granular insecticide, which then should be soaked into the ground. NEXT WEEK: The first of four fertilizings of the fall lawn, choosing daffodils for immediate planting, and fall planting of shrubs and trees. Jack Eden hosts "Over the Garden Fence" Sundays from 10 a.m. to 1 p.m. on WTOP Radio (1500 AM).