In order to have a lush, green lawn, it is extremely important to prepare the soil properly. Good drainage is essential, so the soil must not be compacted. Your soil must also contain sufficient nutrients, particularly in areas where soil quality may be poor, such as the desert southwest; add significant amounts of organic material to build the soil up to the point it can support the growth and maintenance of a lawn.
Turn the soil with a shovel, working across the entire planting bed. Use a rototiller if the area to be planted is large (more than 20 by 20 feet), or to reduce the amount of physical exertion involved. Till down to a depth of 6 inches.
Break up compacted areas. Use a garden fork to break up any remaining chunks of compacted soil. Work the soil until it is loose and crumbly in texture.
Remove debris and rocks. Rocky material close to the surface can inhibit the growth of the lawn's root system. Take out old tree roots or other large pieces of plant material you find.
Add compost to increase the level of organic material in the soil. In poorer quality soil, add up to 2 inches of compost. Add fertilizer with nitrogen, phosphorous and potassium at the dosage recommended by the instructions on the package. Turn the soil again to thoroughly mix the amendments into the original soil.
Install an irrigation system. Run irrigation tubing from the main water source out to the planting bed. Purchase pop-up style emitters and calculate how far apart they need to be for complete coverage of the lawn. Install the tubing and emitters based on your calculations.
Level the planting bed. Rake the area until it is as level as possible. If water collects in lower-lying areas, the grass seeds will drown and you will have to replant. Test the watering system out before the grass seeds are in place. If water pools in certain spots, continue leveling the bed.