Learning how to center and throw a basic cylinder is the single most important thing you need to learn on the potter’s wheel. A cylinder is the basis for all forms on the wheel – learn to throw a good cylinder and you’ll be amazed by what you can accomplish on the wheel.
Learning to throw is hard, though. It takes practice. And then more practice.
Here’s an informative set of instructions from Ceramic Arts Daily. Some images are below, but please refer to the link for more information.
This is another good video with some very clear instructions on best practices when it comes to centering clay.