Here's how I did it, I don't have pictures of the process as this are from a test I made a long time ago. This are two of the only three pieces I made, the third being on a knife I sold, of which I don't have a picture.
I wouldn't like to kill the illusion, but I didn't find this process too difficult. For the cow horn I used a carving too that made a thicker mark, with more depth and less details. Needless to say, I don't like that piece one bit.
For the other piece, this time a piece of axis deer horn, I used a knitting needle that I stole from my wife, and I marked the surface with lines (kinda like scratching the surface), pressing more for the thick lines and less for the thin ones. In the places where I needed to shadow something, I made very light lines, one next to each other, making a very small ammount of pressure.
Of course, before starting to draw, you need to polish the horn properly.
Once the drawing is finished, I passed indelible black ink and while it was still fresh I polished the horn with a piece of clothing to remove the excess. Then I sprayed laque and it was finished.
The entire process took me an hour or so.
I don't really consider it a good work, but I certainly like it more than the other one.
I didn't need to use a stencil or to draw on the horn with a pencil before as I have some skills drawing free hand, most of all if they're sail boats. I don't know if I could do it with other designs, though.
If I ever make another work like this ones, I promise I'll take pictures of the process to post
Regards,
Ruben