There are several ways of cutting shell veneer, anything from craft knife to laser cutter. It depends on the tools available, your experience with them and what effect you are trying to create.
A craft knife or scalpel and a metal ruler, or something similar that is the shape you require - we recommend testing a small piece first. Do this mainly for cutting straight lines. Use on a hard surface, press down firmly on the line or position you wish to cut. Drawing the knife gently across a few times times - just use gentle pressure -, scoring the shell deeper each time until it separates. Repeat several times with increasing pressure as you cut through the shell into the adhesive backing (if it has been applied). This should give the best result.
Scissors - Must be very good quality, very sharp with tight blades. Scissors can be used to cut curves. Using masking tape on the edges may minimise any fractures to the edges. We have found scissors generally cause minute cracking along the edge, which may not be acceptable to your project, depending on intended use.
Other tool to use is a high speed fine tooth blade, e.g. router or circular saw with diamond blade.