With the wheel spinning, gently placing your wet hands around the
sides of the centered piece of clay.
Your thumbs should be on top of the clay and bring them
slowly in toward the center. This will let you determine the
exact center of your centered clay.
When you know the exact center, gently press your thumbs in at that
center. Note that your hands are still supporting the outside of
the mound of clay.
Continue pressing down with your thumbs until you have "opened"
the clay to within about 1/2" of the bottom or the batt that they
clay is on. This 1/2" leaves you enough clay to trim a nice
bottom and foot on your piece later.
Be sure your hands stay wet throughout all of your work with the
clay. If your hands aren't wet, the clay will stick to them
and pull out of its round shape.
When you have gone as deep as necessary, you can use your thumbs
or fingers to open the clay further and create the inside bottom you
want. Still supporting the outside of the clay with one hand,
use the other to move across the bottom from center to wall, opening
the clay further. Note the thumb of the left hand is over the
top of the wall. This helps to keep the wall compressed as you
work. Also, note the way the hands are working together,
touching, to lend strength.