Bex,
The symmetry comes from creating one half and then flipping it over and redrawing it and incorporating the overlap. The I draw the Initial. At this point what I do is take the top half of the leaves and the initial and scan them into the computer. Using Photoshop I then bring in the two elements in as layers. I dupilicate the leaf work and create a third layer. I flip this layer down and snug it up under the first leaf layer. I then cleanup the initial and place it where I want it to be. I also carefully erase the white space inside the top and bottom of the "S" to be able to get and idea of what is beneath/behind the letter. The symmetry is planned and not hard to create. I will then print out the rough copy and then tape it down and place a sheet of frosted mylar over the top. Using an extra fine Sharpie pen I will carefully trace the composite image onto the film. When I transfer to the cased leather I am again trying as best I can to follow my line work and keep the smoothness and cleaness of the design. But this also carries over to using the swivel knife, where I try and refine the lines and fix any possible mistakes.
So to recap...you start with the left half and flip it over and create the right half and having done that you flip it down. So in essence you have taken one image and turned it into four elements and a symmetrical pattern. I also try and incorporate subtle variations. In nature there are subtle variations in leaves and branches and due to Chaos Theory, if you will, symmetry is not always seen.
Hope that helped answering the question.
Storm