After the clay has been prepared and is ready for throwing, the next step is to center it on the potter's wheel. Some potters work directly on the wheelhead, but most use a bat.
For small amounts of clay, you can simply smack the ball or cone of clay onto the bat, coming as close to the center point as possible. Smacking the clay down with some force helps it attach to the throwing surface. Be careful not to have any of your fingers in the way.
For large amounts, after smacking the clay down you may want to pat the clay into the middle before wetting it and beginning to center. To do this, use both hands in opposition to each other with the clay in the middle.
As you center, remember that success during the throwing rests on successfully centering the clay.