Sand down the bone's surface. Use a power sander first, then sand the horn by hand. Start with a low-grit sandpaper, such as 250 or 300, and work your way up to 600 or 700 grit.
Polish the bullhorn with a green honing compound. After the honing compound sets, apply a thin layer of paste wax to the horn. This will give the ink a better surface to adhere to.
Choose a sharp, hard needle. The size of the needle should suit your carving needs. If you do not want to carve with the needle between your fingers, insert it into a pin vice for more control.
Carve the horn. Make dots by pressing your needle into the bone, and scratches by dragging the needle at an angle across the bone's surface. You can also "copy" an existing picture or pattern by laying a piece of paper over the bone and poking holes through to copy the image's outline.
Apply ink to the horn. Soak a cotton swab in ink and run it over the horn's surface. Make sure the ink has a thick consistency. Wipe the ink from the surface with a tissue. The majority of the ink will adhere to the indentations you have made. Once the ink has dried, wipe it down with a damp cloth.
Apply another layer of paste wax to protect your scrimshaw.