Sculpting in clay is a very rewarding experience but it does requires some basic considerations. It is easy to manipulate clay because it is so malleable. It is not so simple, however, to get the right proportions, especially when you are sculpting the human form, or parts thereof. Thus, learning to sculpt lips requires some practice.
Get some good close-up images of lips. Then, select one in particular as your main reference. Choose an image showing the lips in the closed position, so that you can focus on their exact form. Even better, use an actual person as a model as this will give you an ideal three dimensional reference.
Prepare the clay by kneading it in a similar fashion to bread dough. Continue to knead the clay until it has an even consistency.
Shape some clay into a ball about the size of a tennis ball or larger, and place it on the table. Flatten the ball with your hands to create a clay base on which you can sculpt the lips. Do not worry about other facial features. Leave a golf ball-size piece of clay aside to create the lips.
Roll out two small pieces of clay from the extra clay, about the size of large caterpillars for actual-size human lips. Place one roll onto the clay where you want the upper lip to be. The roll of clay should downturn slightly on each side. Use your rubber shaping tool or slide your finger along the edge of the rolled clay to blend it into the base on both sides of the roll. Create a gentle slope by gently shaping the clay from the base up toward the middle of the roll.
Finish the top half of the upper lip by using your finger to create the vertical trough in the middle of the upper lip that runs up toward the nose. Focus only on the mouth region and imagine the rest of the face around it. Examine the reference image to shape this accurately. Use your finger with some water on it to create a smooth indentation. Shape it so that it is about the thickness of your little fingertip.
Carefully shape the lower--or inner--portion of the upper lip. Note that it has sharper angle than the top edge. Use a knife to shape the lower half of the upper lip. Gently run the blade along the lower side in an almost vertical position. Re-shape, if necessary, until you achieve the correct angle. Use a damp sponge to smooth the edges, creating a realistic bevelled surface and to clean any tool or finger marks.
Shape the other roll for the lower lip before putting it in place. Note the line that separates the fleshy upper portion from the side that curves into the face at the bottom.Look at the image and recreate the curve of the lip along that line. Note how it curves slightly downward toward the chin from the corners.
Make sure that the inside (upper-half) of the lower lip has a bevelled (or inflated) shape as it curves up into the mouth. Shape the roll so that it matches the contour of the inner upper lip. Shape this upper half edge of the lower lip into a horizontal line.
Place the partially formed lower lip into place underneath the upper lip on your base. Push in gently as you attach this piece to preserve a crisp line where they meet and to avoid distorting the shapes you have sculpted.
Slide your finger or rubber shaping tool from the the top edge of the lower lip down to gently blend the clay of the lip into the clay of the base. Turn your finger as you blend to shape the side as it curves into the lower portion of the face.
Smooth the surface using a damp sponge until the top and bottom half of each lip are completely blended into the base and look finished. Avoid running the sponge over the finished inside area of the lips as it will remove detail.
Remove any tool or fingermarks using the sponge once you are satisfied with the sculpture.
Use the same technique when creating lips on an actual bust. Practicing will drastically improve your technique.