Assuming you would like tips on how to paint a mermaid tail:
One way to approach this painting project is to first sketch your design onto the fabric with a chalk pencil or other light-colored, water-soluble fabric marker. You can also create an applique for your design using fusible webbing and iron-on adhesive. Another option is to use stencils to apply your design directly onto the fabric.
Once you have transferred your design onto the fabric, it's time to start painting! To ensure even coverage, it's best to use a small foam roller or brush designed for applying paint to fabrics. Simply dip your roller or brush into the paint, then apply it evenly over the surface of your mermaid tail. You may need several coats of paint to achieve desired opacity. Let each coat dry completely before applying the next one.
Once you're happy with the color and coverage of your painted mermaid tail, set it aside to let the paint cure completely according to manufacturer's instructions (this usually takes 24-48 hours). Once cured, your tail will be machine washable - just be sure to turn it inside out before washing and drying.