Apply a 2cm layer of good quality compost or topsoil to replace the healthy organisms that have been disturbed by loosening the soil
Rake the ground level
Roll open the instant lawn and pack it closely together, to avoid gaps
Use either a roller or a stomper to make connection between the lawn and the ground below. This is not about compacting the ground, but rather to ensure there are no air pockets. You can use your feet to stomp it or even the back of a spade if you do not have a roller.
Water! Give the grass a good soaking
Pat yourself on the back for a job well done and read the maintenance guidelines for what to do next!