What preparation is needed to apply knockdown ceiling
I assume that you have a fairly rough surface at this point. When I refurbish texture over a large area I do a full skim finish first. If you want your texture to look new and professional you can't go over a mess. Standard or topping joint compound work well.
So, skim, scrape any ridges, and skim again. Repeat until it's like new, then sand lightly.