Another way: sketch or transfer a design on your shell, and use a air or electric grinder, (Fordom and Dremal are two that I use), Diamond and carbide bits work very well, then just trace your design with your grinder. you can fill etching with paint, ink,etc..Good-luck Etching Seashells A man I knew used pins, nails, small knives, and then rubbed the etching with India Ink. Depending on what kind of shell you are etching you must be very careful. My suggestion is to make sure you use a variable speed dremel Tracing your pattern with a fine permanent ink marker is your best bet to start out. Good luck.
I would experiment with a dremil they have pen like dremil tips :) but would work with a varable speed one so you have more control.