Establishing a new lawn takes advance planning and work. Sowing seed or laying sod is only the final step.
Start by determining what type of lawn is needed. Will it be heavily used for group sports, play by children, or exercise for dogs? Or is it intended not for foot traffic, but simply as a lush, fine-textured green plot in the overall landscape? Once you've made these decisions, choose the appropriate cool-season or warm-season grasses that can provide the characteristics you need. The most appropriate choices will probably be those stocked by local nurseries or lawn specialists. Read grass-seed package labels and descriptive information; ask for flyers or brochures that describe the grasses in various sods.
When preparing the area to be planted, make sure it has a gentle slope away from buildings and other areas that could be damaged by standing water. In general, allow a 1-foot slope for every 100 feet of distance. As you measure for slope, you may find that some areas are higher or lower than others; grade these for an overall even appearance. If you need to bring in additional soil, buy the same type as the existing soil (to the extent this is possible) and mix it with the existing soil as you work.
Test and amend the soil as you would in any other garden area. Because grass forms a thick mat about 1 inch high, the prepared planting area should finish out at about an inch lower than surrounding areas.
If you are installing an underground sprinkler system, allow enough time in your schedule to design it carefully for complete, even coverage; or have a licensed landscape contractor do the design and/or installation for you. The system should be installed after the lawn area has been graded.
Seed or sod?
The greatest advantage seeding has over sod is cost. Though improved growing, harvesting, and distribution has made sod less expensive than in years past, seeded lawns remain much cheaper to plant. And while sod offers a wider choice of grasses than it once did, seed still provides the most variety. You can easily find hybrid seed mixtures that thrive in shade, for example, but these are harder come by in sod. Sod also has occasional problems with bonding to the soil beneath; if it fails do so properly, you'll get a shallow-rooted lawn at best--or, at worst, one that completely fails.
On the other hand, many gardeners can't stay at home to keep a seeded lawn constantly moist for weeks, and not everyone has an automatic sprinkler system that allows for watering several times per day. Sodded lawns must be kept moist, too, of course, but they don't dry out as fast as seeded lawns; watering just twice a day (before and after work, for instance) is often enough to do the job.
Sod also provides an instant reward for your labors - a morale booster if the entire garden is brand new, with only small trees and shrubs dotting the landscape.
Starting from seed
Seeding applies primarily to cool-season grasses; most warm-season kinds are started from sprigs or plugs.
Lawns started from seed are best planted in fall, early enough in the season to give the grass time to establish before cold weather comes. The next best time is spring, after all danger of frost is past and before weather turns hot.
When you prepare the soil, don't cultivate it too finely - it may crust, forming a hard surface which emerging seedlings cannot penetrate. Ideally, aim for pea-size to marble-size soil particles. Do final leveling with a garden rake.
Pick a windless day and sow seed evenly, using a drop or rotary spreader.
Apply a complete dry granular fertilizer, also using a spreader. Several manufacturers offer fertilizers formulated especially for starting new lawns.
Cover seeds by dragging the back of a lightweight leaf rake over the area or applying a thin (1-inch) mulch. Mulching is the better option if you expect hot, dry weather or drying winds. Use an organic mulch, but not peat moss or sawdust--both of these tend to crust over, making it hard for seedlings to penetrate them. Note that it's not necessary to roll the new lawn's surface with a water-filled roller. Doing so can actually inhibit germination, since the roller packs down the soil surface and causes it to crust over.
Water thoroughly, taking care not to wash away the seed. Then keep the seeded area moist for about 3 weeks or until all grass is sprouted, watering briefly (in 5- to 10-minute spells) and frequently. You may need to water 3, 4, or more times a day during warm periods.
Mow for the first time when the grass is one-third taller than its optimum height. Mow slowly to keep from disturbing the barely set roots. After the initial mowing, continue to water frequently; the top inch of soil should not be allowed to dry out until the lawn is well established (this usually takes about 6 weeks and 4 mowings).
If weeds emerge, don't attempt to control them until the young lawn has been mowed 4 times. By this stage, many weeds will have been killed by mowing or crowded out by the growing lawn. If weeds are still a problem after 4 mowings, many gardeners prefer to treat the lawn with an herbicide; unlike hand pulling, it kills weeds without the risk of disturbing the root systems of the grass.
Try to avoid walking on the lawn too much during the initial 4 to 6 weeks.
Starting from sod
Sod lawns can be started almost any time of year, except when weather is very cold. It's also best to avoid installation during a summer heat wave.
Water the planting area thoroughly the day before the sod is delivered.
Time the delivery of sod so you can sod the area in a single day, beginning early in the morning.
When you lay out strips, stagger them so ends aren't adjacent; butt sides tightly together. Use a sharp knife to cut sod to fit it into odd-shaped areas.
Roll the entire lawn with a roller half-filled with water to smooth out rough spots and press the roots of the sod firmly against the soil. (Rollers can be rented at garden and tool supply centers.)
Water once a day (more often if the weather is hot), keeping the area thoroughly moist for at least 6 weeks.
Mow for the first time when the grass is one-third taller than its optimum height. When mowing during the initial 6 weeks, be very careful not to disturb the seams. Also try to avoid walking on the lawn too much during the initial 4 to 6 weeks.
Starting from sprigs or plugs
Many warm-season grasses are sold as sprigs or plugs. A sprig is a piece of grass stem with roots and blades. A plug is a small square or circle cut from sod. Early spring is the best time to plant sprigs and plugs.
Sprigs are usually sold by the bushel; the supplier can tell you how much area a bushel will cover. The fastest way to plant them is to scatter them evenly by hand over the prepared area, then roll them with a cleated roller (this tool is usually available for rent from nurseries that sell sprigs).
Plugs are usually 2 to 3 inches across and are often sold 18 to a tray - enough to plant 50 square feet. Plant the plugs in the prepared area, spacing them 8 to 12 inches apart.