Saw this photo on the net and decided that this was for me. I don't want to do the ENTIRE backsplash with shells. I am getting antique white cabinets and hope to get terracotta colored countertops (Juparana Bordeaux?). I am getting a soft beige tumbled marble backsplash but, instead of a mural behind the stove, I want to make a bordered shell accent (between 18x12 to 24x18 - I have to see when things are in place). I think the colors in the shells will echo the warm brown/terracotta colors in the countertop. I need to know:

How do I go about this? What do I put the shells on? Do I just go for it and lay mastic on the wall and arrange the shells and pebbles there AFTER the tiles and border are up? Do I find a base of some sort (I thought of wood but it might warp). NO idea how to proceed.

ALSO...any ideas how to attach accents or borders to tie this together along the rest of the counter? I'd love to create a 2-3" strip running through the tiles somewhere but - again - no idea what to attach the shells and pebbles to so that I could handle them in rectangular pieces for that kind of a strip running through the tiles or along the base. I hope I'm making this clear!

Help and how-to suggestions most welcome with thanks!