Mermaid Tail Diy Costume . Fold down the top edge of the skirt by ¼ to ½ inch (0.64 to 1.27 centimeters). Prep the green material for the tail. Benzie A fanfare of felt. Mermaid Tail DIY Mermaid costume diy, Diy from www.pinterest.com No sew mermaid costume diy: When tracing around, make sure. In order to gather the bottom piece, sew a basting stitch along the top edge.