Shells can easily be used to 'decorate' flower pots - here are a few examples of some in my garden.
The upright pot is the easiest - I used ready-mixed tile adhesive. The adhesive not only holds the shells in place, but covers the entire background (or surface area) of the pot in a neutral color.
When working with a 'long' container, such as that in this photograph, I found that was best to deal with the ends first. Once that had been accomplished, I rested the container on its long side, and 'decorated' the front. The back was not decorated, as it is not on display.
The rock in the corner of the photograph had a nice shape, so I decided to paint it, and then added some shapes using a silver metallic permanent marker.