This post is sponsored by JOANN but all pictures and words are mine. This may contain affiliate links at no extra cost to you. Thanks for supporting Sisters, What! 
I bought the fabric months ago, but it took me all summer long to finally get started on it. Before I made it, JOANN came out with some glow in the dark glitter tulle and knew I needed to finally make a skirt. The tulle is fun and actually glows really well. The tulle is perfect for the bottom of the tail and adds glitter everywhere she goes.

How to Sew a Little Girl Mermaids Costume

 

Step 1: Cut Fabric

Step 2: Make a casing for elastic

 Step 3: Ruffle the Tulle

First cut the purple tulle. I did double the skirt width by 6", so 64"x 6". I did three layers for the purple of the tulle. If the tulle isn't wide enough, then sew two together.

For the green tulle, I did 64" x 8"

Now ruffling the tulle: An easy way to ruffle tulle, is to use floss. When you are ready to ruffle it, place the floss near the edge of the tulle and set your zig zag stitch at wide stitch. Carefully start sewing on top of the floss without sewing the floss. Be careful not to pull the floss through as you sew.
 After sewing, take the floss and pull it through until it is ruffled.
 Align it with the bottom of the skirt panel laid out and refit the ruffles to match the mermaid skirt.
 Do the same with the green tulle.

Step 4: Sew ruffles to skirt

First place the purple ruffles on top then place the green on right top of mermaid skirt . Then slowly sew it in place. Flips the ruffle down and make sure all is sewn well in place. Then carefully unpick the floss.

Step 5: Sew up side

Next place the skirt right side together and sew up the side start right BELOW the band at top.

 Step 6: Add elastic

Use a safety pin and attach to elastic. Carefully guide it through the casing. When it comes through, sew the elastic through and sew it with a zig zag stitch. Sew the ends of fabric together after finished.