Assemble a number of baby photos to learn baby anatomy and proportion, so you can accurately portray them. For example, in an infant the head is usually one-third of the body length, where as in an adult it is typically one-sixth. Infants also have feet that are about exactly half as wide as they are long and hands that are shorter fingers than palms, whereas bigger children have same size or longer fingers, than palm. Small details like this lend credibility to your work.
Start with a single block of clay and remove chunks of clay from it until you have the general shape of a baby. Remember, about one-third of the block should be devoted to the head, the rest for the torso and limbs. Use your clay knives to cut off chunks of clay, shaping the remaining clay as you go. When you are finished you should have a sort of ball shape at one end, and a longer oval at the other.
Cut two arms and two legs into the torso area. For the legs, simply cut strait up the center,directly opposite the head, to about one-third of the way up the oval -- not the entire clay block. The arms are a bit harder. Cut wing-like flaps up the side of the oval starting from where the leg split ends. These wing-flaps should be one-fifth the width of the clay block's thickness from side to side which is essentially the same ratio a child's shoulders and chest would occupy.
Shape the clay using your fingers and the knives, until it takes on a more realistic, although not particularly detailed form. Start at the feet end by molding the general shape of feet, then making four small slices to the ends to more easily make toes. Babies ankles are typically quite chubby, so you wont need to thin them like you would an adults. The calves are usually short and fat, with dimpled knees and round, jiggly thighs, so it is okay if the figure is not smooth and sleek. It is supposed to be dimpled and round.
Smooth out the tummy -- which is typically not dimpled, and is the same width as the chest -- curving into a flatter chest. Then add some of the clay you removed earlier to create a diaper if you wish.
Shape the wing-flaps into arms, paying special attention to the visible sides of figure -- the baby's shoulder blades essentially. The arms are also dimpled, like the legs, although they are about half to one-third as thick as the legs. The shoulders should be small, narrow and very rounded, almost curving inward. The neck is typically thick and short to the point of invisible, since most babies hunch their shoulders and curve in towards their tummies.
Ignore the hair when creating the baby's head. Hair does not look good made in clay and many babies are born hairless, so your sculpture will not suffer from having no hair. Shape the face paying close attention to proportion. The mouth is small, but the nose typically slightly smaller, wide and "nubby." The eyes look disproportionately large. Babies have a very large, wide forehead and a smaller, pointer chin. The cheeks are quite wider than older children's and usually much rounder.
Detail your work using a round pointed clay tool. This tool is used to sort of scratch the clay to add details like creases around the knees, the edges of toe and finger nails or even to add eyelids. Press lightly, keeping in mind that babies have creases, not wrinkles. Allow the clay to dry for an hour or two, or however long it takes to becomes cool, smooth and firm, but remain workable.
Dampen a soft sponge and squeeze out the excess water. Gently pass the sponge over the clay to smooth the surface, remove your fingerprints and soften the look of the piece. Be careful that the sponge isn't too wet since that can ruin your work. Also avoid pressing roughly which will erase details like creases. You may still have to re-add a few of the details, do so while the surface is still damp from the sponge.
Allow the figure to air dry or use a kiln as directed on the clay container. Sand the surface or paint if desired.