For start you need to fill the area where you want a lawn in the first place. For that you need at least 3 feet depth to shrubs, plants and small trees to grow easily. If you plan to plant a tree the depth should go deeper. But it is recommended that to make only those areas deep where trees will be planted as it will create a great toll on your budget as soil composite is not cheap.
After prepping the area where you will be pouring the in soil composite, make sure that its free of pest and insects. For that you will need to put chemicals and pesticides under the composite. So this is the best time to cover the whole area with pesticides. There are many brands out there which have doses level recommendations of their own and types of solutions also. Some are premade liquid solutions and some require mixing it with water. Choose whatever is best for your budget and lawn application.
These chemicals help with unwanted insects and pests to come on the lawn. Some have said that it keeps the level of mosquitoes and flying pest away too which is a blessing if you want to enjoy your lawn at night in a city like Karachi where mosquitoes are a big problem in outdoor or indoor gardens.