Hi, welcome to GoY, I presume you mean you have top dressed you existing lawn, and now want to put turf down on top for some reason, if you have a problem with weeds, or drainage on your lawn, and you put turf down, then the weeds will come up through the turf, if it's a drainage problem, turfing won't solve the problem, you will still have a drainage problem.
Can you say why you want to put turf down please, Derek.

If you have put top dressing of any reasonable thickness on bare ground you would need to firm it before adding turf