Purchase seashells from a craft store or online vendor such as Seashell World.
Lay a large towel on a table or another flat surface. Spread the seashells onto the towel. Leave three inch between the seashells and the edge of the towel. Lay another towel over the top of the shells and fold the edges together.
Roll the rolling pin over the shells inside the towels until all of the shells are crushed. The slow movement of the rolling pin should ensure that the seashells are crushed evenly.
Peak under the towel to check on the size of the shells. If they are not crushed to the desired size, roll the rolling pin over the length of the towel again.
Remove the top towel and roll up the second towel carefully in order to pour the crushed shells into a bucket or another container.