Preparation of soil before laying turf

Hi,

 I have recently moved into a new build and would like to lay turf. I have read plenty about how to do it but I'm still unsure about what I have to do to the soil under the turf before laying. So my questions are:

1. How deep do I have to dig down to prep the ground for the turf?

2. What is exactly meant by 'dig down'? Does it mean break up the existing soil for example with a spade (it is not a very big area) into small pieces?

3. I have read that I need quality top soil under the turf. So how deep should that be (excluding the soil that comes with the turf)? Should I mix it with the existing soil and if so how? Break up the existing stuff and then pour the good topsoil I buy on top? Or turn the existing soil over with new topsoil so it mixes up?

As you can see I am a complete novice so I want to get this right and not end up with a dead lawn through committing some rookie error! Any help appreciated!