As a ceramic artist primarily working in figurative sculpture, I sculpt a lot of hands. When it comes to sculpting fingernails, I have found some short cuts. My long-time friend Judy Mooney showed me an easy method for sculpting nails on a life-size hand: using press-on nails. This method impresses the shape of the nail bed into the finger without sculpting by hand. Press-on nails come in a multitude of sizes and shapes, so you will need to pick the ones that best fit the hand you are sculpting. (They even come in kid sizes for those smaller hand sculptures.)
After the general shape of the hand and fingers are sculpted, and the clay is leather hard, simply press the plastic nail into the clay on each finger (1), making sure that they are secure and that the clay conforms to the form of the press-on nail. This will create a bit of a suction. Do this to all of the fingers and then double-check that they are in place by gently moving them from side to side (2), which will make a deeper impression in the clay. The side edges of the plastic nail should slightly disappear into the clay.
Carefully remove the nails by lifting upward on the overhanging free edge of the nail. Use the cuticle end of the plastic nail to form a groove in the clay where the fingernail ends. Use a wet paintbrush or sculpting tool (3) to smooth over the surface of any marks made by releasing the nails from the clay. If sculpting longer nails, extra clay may need to be added before application.