How is the ceiling sagging and by how much, etc.. Your extreme economy of words doesn't give us much to go on. I would probably find the low spot and strike level line based on that. Options are to leave a gap or fill it with a moulding or trim. Large sag will show less if gap or trim is at least a few inches wide. If you have a LARGE sag then you might look into why. Is the drywall falling off? Is your ceiling going to fall in? If the floor has a similar sag then your sill may be rotted and failing.