I draw moulding lines by hand in pencil. There shouldn't be a straight line on a stock, with the exception of the top of the forearm. So, the moulding lines shouldn't be straight. Once I have it right, I cut in most all moulding lines with a Sloyd knife that has had all but about 1/2" of the edge at the tip ground off. That way, I can grip the blade right up to the tip and put my thumb at the back of the tip. I walk that rounded tip down the line rolling it back to front. I also use it to clear the background adjacent to the line I just cut, holding it the way I indicated.
For the forearm, I generally use a scratchstock to apply the moulding, at least to the upper forearm. The rest of the moulding is cut as above.