Gluing shells onto a frame made of wood covered in varnish.
It depends. (Sorry, it's an engineer's answer.) If you want to keep the original finish with the dark wood, there might be a wax varnish stripper at the hardware store. It you don't want the original finish, I would sand it down first starting with a coarse sand paper working to fine. I would prime/paint it with a matte finish paint, and you could probably get one of those sample size tubs of paint for about $2. The matte paint or even eggshell paint would allow for better adherence. As for glue, there are several options to choose from. There is hot glue, wood glue and construction adhesive. The construction adhesive may turn colors though. Like I said though, it all depends on what you want from that original frame.