For best result, rototill or spade the area to a depth of 10 to 15 cm. (4 to 6 in.). Eliminate drainage problems by having soil slope away from foundations, etc.
Soil test your lawn area with the assistance of qualified service in your area. Then rake in fertilizer, lime, peat, compost, etc. as need to a depth of 7 to 10 cm. (3 to 4 in.). Rake and smooth the soil removing rocks, roots, and large clods. Roll the area lightly with a lawn roller 1/3 full of water. This will firm the soil surface and reveal low areas that need more soil. Keep the grade 2 to 3 cm. (1 in.) below sidewalks or driveway.
Water the prepared area to settle soil and provide a moist base for turf. Moistening to a depth of 15 cm. (6 in.) is recommended for most soils.
The sod will mirror the ground. Be sure to have all settling and smoothing taken care of before laying the sod.