You think you know how, but maybe this will make it easier...
You will need one apple and a paring knife.
Cut the apple in half, and then half again.
Starting from the bottom of the apple remove the core.
The core will pop out smoothly. Why? Because of the angle of it from the bottom to the top. In the picture below, the bottom of the apple is at the left. Notice the angle of the core? It enables you to slide your knife in and remove that core with ease!
Try it and let me know what you think.