Countersink first, drill the pilot hole second. That may sound backward, but it’s the easiest way to ensure a perfectly smooth countersink. I used to drill the pilot hole first on the drill press, but if that hole was relatively large or the wood quite dense, the countersink bit would inevitably chatter and make an ugly, rough surface.
One day I tried the countersink bit first with no pilot hole. What a difference! It made a perfect conical depression. It’s easy to locate the hole because a countersink bit has a sharp tip. Centering the pilot hole is easy, too. A twist bit practically positions itself in the hole’s bottom.