Looking at your question got me to thinking. I wonder how much drawing they even did, if at all. Maybe some rough outlines but I doubt more than that. I have a friend that is a professional wood carver (not on guns). He does very little or no drawing or sketching before he starts, but he is highly trained, and has an artists eye that I envy. He works quickly roughing out the outlines and design, then spends more time on the details as he refines and finishes the carving. Remember, the old time gunsmiths were professionals too. Time was money and they had been trained far more than most, if not all of us. Plus, many of them used similar designs on most of their guns, they would have had the basic design well fixed in their minds before they even started carving. However, like Dave says, they did have pencils, chalk and even charcoal could have been used also.
I am not trying to say they never did draw their designs, I don't know, but I think sometimes we bring our modern concepts of how things should proceed. We tend to forget that in those days, this was something they did day in and day out and had spent years in training to learn. It was work, not necessarily something they did for enjoyment. Also, today many of us are trying to closely copy a specific design or maker which would not have been a factor in those days. I doubt that Armstrong, Rupp or Dickert, were very concerned if their carving did not end up exactly as they planned it initially.
