You can make most of your mermaid costume out of common household objects, but the tail poses challenges. A paper mermaid tail completes your look, doesn't cost a lot and is fully customizable. Make a matching front and back tail for a more authentic costume -- and less mobility -- or stick with one. Attach the tail to your clothing with safety pins, and sit carefully lest you rip your handiwork.
Find a piece of paper large enough for your tail. If you're making a tail for a doll or a child, a piece of construction paper or poster board will do fine. For an adult-sized paper mermaid tail, look for roll paper, such as art paper or craft paper. Roll this paper to the desired length and cut a straight line to remove your piece from the roll.
Draw a long pie wedge on the piece of paper, which forms the basis for your mermaid tail. Make the top of the pie wedge as wide as your waist, the child's waist or the doll's waist. Use pencil so you can erase if needed.
Add tail fins to the bottom of your pie wedge by making two smaller triangles that extend from the bottom of the pie wedge. Either keep the lines angular or use curved edges.
Cut the tail out using scissors.
Draw scales on the tail using your pencil. These look like small lowercase "u" or "v" shapes and extend across the whole tail.
After cutting out your tail and making scales, decorate it to personalize. Use markers or paint to tint the tail, or use glue and glitter to highlight scales, fins and other features.