The best way to do this is by crushing them by hand into smaller pieces. You can also use a blender, coffee grinder, or food processor to grind them up into fine powder.
Add the eggshell powder to your outdoor compost pile or bin along with your brown and green materials. Make sure to mix everything together well, and keep your compost pile moist and aerated.
Crush the eggshells into small pieces. You want the pieces to be small enough that they won’t hurt your plants, but big enough to create a barrier that pests will have trouble crossing.
Over time, the finely-ground eggshells will break down and lose their effectiveness, so it is a good idea to reapply them as necessary to maintain pest control.