Before laying lawn the area should be thoroughly prepared. It's best to start this operation as soon as the ground is dry enough to cultivate and if possible allow a period of fallow before laying the grass. This allows any remaining perennial roots to sprout and weed seeds turned up during digging to germinate, so that they can be removed or killed off. Repeat the process a few times to give the new grass the best chance to grow uninhibited and reduce the development of a weedy sward later.

The site will probably need a certain amount of levelling depending on the finish required. For a formal lawn it should be reasonably flat and the humps will have to be removed. Shallow bumps and dips will be sorted while digging, but large changes in levels that are deeper than the topsoil layer will require a bit of engineering. The soil will have to be removed and the subsoil moved around to the required level before the topsoil is replaced.
Check to see if there are any drainage problems, soggy areas will need some sort of treatment to give a satisfactory lawn. In the domestic situation, it may be worthwhile to start with a layer of 19mm gravel covered with some grit to 'blind' it if you are bringing in fresh soil, particularly if the subsoil is dense clay which will not drain quickly. For a larger area you may need to sink some small drains filled with stones into the clay below the gravel, to carry excess water away (sometimes called a French Drain). If there is no outlet to allow it to drain from the site, a soak away consisting of a pit filled with more stones should cope with any water which collects. After going to this length to ensure good drainage the topsoil must have a good structure as well so if it has a high clay content, incorporate plenty of grit and organic matter. Sandy soil will need organic matter to improve retention of moisture and nutrients.
With the levels and drainage sorted, fork over the soil removing all perennial weeds with their roots. If they are dense the best thing is to skim off the surface first and any remaining roots will be found when digging. In a large area the skimming and digging can be done with hired mechanical equipment, but the advantage of hand digging is that you can be more thorough in finding roots and stones. Any stones larger than 25mm in diameter should be removed - this will become an obvious step if you later try to spike the lawn. They can be used to form the base of a path or to fill a drain. If the soil is not in good condition this is the time to add some organic matter. To improve drainage and aeration add some grit by spreading a 30 to 50 mm layer and forking it in. The soil will be loose and full of air pockets, these need to be compacted otherwise later settlement will result in a bumpy finish. A roller is useless for this as it is too wide, missing small voids; the most efficient method is tramping the whole surface, putting the weight on the heels and shuffling back and forth. Onlookers may find it amusing, but this a tried and tested technique. When the compacting is complete rake to a smooth finish, filling small depressions to produce a seed bed, or to be ready for laying turfs. Small stones and large crumbs of soil will be pressed into the surface later, but if there is a lot of debris it should be removed.

Irrigation is a contentious subject and in the present situation of water shortages and high water charging, watering the lawn is a luxury, but it does not need to be watered after it has been established. Too much irrigation causes the roots to remain close to the surface so they are unable to reach water lower down. Left to develop normally they will grow down further and are better able to withstand a dry spell. The first thing to do in dry weather is to allow the grass to grow longer, the deeper sward keeps the soil cooler so there is reduced loss by evaporation. If water is in extremely short supply, it should be kept for more important plants which do not recover. Even after apparently dying away much of the grass will recover and can easily be re-planted if necessary.