Yes. You want to spread each card individually. Perhaps you should start practicing your spread. To be honest I too learned to cull from EX's recommendation, "Cullfather," and I haven't read "Card Colledge." I am not exactly sure how the move is described in that book.
If it makes you feel any better I found culling, and making it look natural, one of the more difficult moves. I have spent a lot of time on it and there are still many culling applications that just don't look good enough for me to try in front of an audience. Don't get discouraged. Just keep at it an sooner or later it will click.
I am also finding it pretty challenging. The cull itself is now better, since I've been practicing it for about a week and a half or two, Card College says that you have to use your left thumb to spread the cards, then in the next paragraph it says that you have to "place the pad of the left thumb onto the face of the card directly above and to the right of the" card. As I am spreading, I naturally use my thumb to push the cards, but since I push a lot of cards, it is possible that one of the cards that I am pushing with the pad of my thumb is the selected card. Has this never happened to you? Am I spreading the card wrongly?