Bring the ground to a fine tilth by rotavating to about 100mm. Roughly rake and grade the ground to levels required. Shuffle over the whole area to consolidate. Rake again to remove stones/debris over 25mm, making up low-ways and skimming off high spots as you go. Broadcast grass seed evenly at about 40gms per sq.m. and Growmore base fertilizer at recommended rate (cant remember what it is, but it'll be on bag!). LIGHTLY rake over whole area again and leave for nature to take its course. If youre in the UK, there should be enough rain to get the seeds started and they will be up in 10-14 days. When the grass is about 70mm cut by rotary mower with a roller to about 50mm(removing the cuttings). Providing you've had some rain, then cut again every 5 days or so, reducing the cut to about 25mm after say the third cut, when the lawn should be OK for light use. Only water if the grass looks to be drying out.