Just the way I do it but I'm sure most others do it the same, find the center line of the shell, draw your design on one side, trace, flip over the design and copy to the other side. Then just use that template for the other shells. Some shells you can just sit over the top of the one with the design on and trace it that way but alot of shells don't fit flush enough to do it.
By the way, nice design Mike
