Cut or carve the long top edge of the headboard into repeating, rounded triangular shapes, like cresting waves, evoking a mermaid's traditional ocean habitat. Apply swirls of paint in different shades of blue until the headboard is covered. Add white tips to the top of each triangular "wave" to evoke whitecaps. On the large, flat surface of the headboard, place decals of the undersea creatures like fish and turtles with whom a mermaid shares her watery home.
Craft a clever set of sheets for a mermaid bed by painting a mermaid torso and tail on a blue top sheet. That way, anyone tucked under the sheet will look as if they are a mermaid. This is a particularly appropriate way to tailor a mermaid bed for an imaginative child. Alternately, you could cut, sew and stuff green felt into a pillow shaped like a mermaid's fish-like tail to make a useful mermaid bed accent piece.
Add a larger-than-life silhouette of a mermaid to a bed canopy to create an eye-catching mermaid accent. Obtain a pre-made canopy in an oceanic color like turquoise or deep navy blue. Make sure the canopy is fitted to the bed. Cut the silhouette of a mermaid from a large piece of felt or other fabric that does not fray at the edges and sew the silhouette in the center of the canopy.
Add a dramatic painted border of sailing ships to the side and foot portions of the bed frame to allude to the mythological tradition of mermaids calling sailors to watery deaths with their voices. Possible approaches to painting ships on the bed frame include cutting a large sheet of sponge into a stamp (dip the stamp in paint and press it to the frame), painting the ships individually by hand, and making or purchasing a ship stencil over which paint is applied.