How do you grow horns?

It is possible for human beings to grow horns. They are, specifically, called Cutaneous Horns and occur more often in the elderly than anything else. Causes for these growths are, as far as I know, completely unknown. Most times, these growths can actually be cancerous.

Two well known cases are as follows:

Zhang Ruifang, aged 101 (living in Linlou Village, Henan province, China), has grown a cutaneous horn on her forehead, resembling what those who have examined her and her family call "Devil's Horns." Notably, this growth has expanded to reach a total of 6 centimeters in length. Another is forming on the opposite side of her forehead

Madame Dimanche, called Widow Sunday, a french woman living in Paris in the early nineteenth century, grew in six years from the age of 76 a 24.9 cm (9.8") horn from her forehead before it was successfully removed by french surgeon Br. Joseph Souberbeille (1754-1846). A wax model of her head is on display at the Mütter Museum, The College of Physicians of Philadelphia, US