Rich, after I have my drawings and layout decided on, I free hand transfer it onto the horn with a #2 pencil. I squirt it with a little hair spray to help keep it from smudging while I carve it. To some extent you can use dividers and compasses to help accurately transfer your drawings too. That said, though I've never tried it, I've been told that some Tansel horns have little stipple marks presumably left from using a sharp awl or big needle tip to transfer the rough outline of a drawing done on paper to the horn.
Tim A