Kill off all existing grass and weeds with a non selective weedkiller such roundup. Allow existing growth to die off over a period of 7-10 days for the roundup to work through to the roots. (For best results ensure round up is applied on a dry day and as early as possible in the dayto avoid rain and dew diluting the active ingredient) After this period the existing lawn have turned yellow/brown and all weeds and grass will be dead.
Using a spade or garden fork dig the soil to a depth of approximately 25cm (10 in) and break up large clumps pick out large stones and other debris from the soil. Allow time for the soil to dry and become more workable normally a couple of days (new housing estates can tend to have very poor soil and contain a lot of builders rubble just below the surface this should all be removed also)
Replace any areas of removed debris or rubble with fresh topsoil and break down soil to a fine workable soil using a rake in a back a forth motion this will also level the ground filling lower lying parts of the lawn.
Rake and firm the soil to complete the seed bed. Firming the soil can be done by rolling with a light garden roller or simply walking on across in a flat sole shoe. Area will be firm under foot at this stage.
On an ever so moist surface sow seeds by hand or using a drop spreader evenly over the entire area rake over lightly and roll or walk over area again to press seen into the soil.
Water in thoroughly being careful not to wash away the seed. Continue watering in dry conditions until the grass is well established check soil for moistness in you new lawn daily.
Your new lawn should appear within two to three weeks.