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.