To use it: "Grease and flour the heating core on the inside and the outside, place in the centre of the pan (with the opening facing up), fill with batter to the same level as the pan, and bake. After removing from the oven, remove the cake from inside the core, plug the hole in the cake, trim, and ice."
To use the flower nail, you place it in the middle of the pan with flat side down, and the point sticking out of the batter. I have used one of these, and can attest that it works.
Same for the cake strips/ cake bands teagal mentions. I routinely use those and get much less doming in my cakes, so there's less to trim off when layering.