Purchase medium-density fiberboard and have it cut to the appropriate size for your bed.
Measure the width and height of the fiberboard. Transfer the measurements to a piece of cardboard. Cut out the headboard-shaped cardboard with box cutters. This will serve as a guide for the placement of your shells on the actual headboard.
Arrange your seashells on the cardboard. Move shells around as desired to create the design or arrangement of colors you want.
Place the fiberboard on a flat work surface. Work on the headboard one section at a time. Apply craft glue to the back of a shell. Press the shell onto the headboard. Repeat this until you've glued all the seashells into their places. Allow the craft glue to dry for 24 hours while the fiberboard lies flat.
Spread grout over the spaces between the seashells using a trowel. Smooth the grout gently to avoid breakage of fragile shells. Make sure to fill in the cracks to help hold the shells in place.
Wipe excess grout off the seashells with a damp sponge. Rinse the sponge with water frequently. Wring it out to remove excess moisture. Continue to wipe until excess grout is removed and the shells don't have an opaque grout film on the surface.
Allow the grout to dry and harden for 24 hours before installing your headboard.