Walk the area four weeks before planting the grass and remove any sticks, stones or other debris. Pull large weeds out by the roots.
Level the area as much as you can by reducing high spots and filling in low spots.
Rototill the area in a horizontal pattern, and then again in a vertical pattern. If there are any trees in the immediate area then only till near them to a depth of 6 inches.
Rake any stones or other large debris that surfaced during the tilling.
Water the area as if you were growing grass. Wait two weeks for the weed seeds in the soil to begin sprouting. Rototill across the area until the weeds are gone.
Wait another two weeks for any remaining seeds to sprout. By tilling them under again you should have removed virtually all the weed seeds available to sprout.
Add any soil amendments such as compost, lime or new lawn fertilizer just before the last rototilling. Spread the amendments evenly across the area and ensure they are tilled in at least 6 inches.
Rake the area smooth and then roll over the area with a lawn roller to slightly compress the soil.