I used to have a fishtail skirt in my yoof! Of course I got googled that and got directed to lots of mermaid dresses where the flare is achieved with gores.
You will get the flare you want by drafting a circle, though you will not get the concentric rings in the dress that
@SewRuthieSews posted. I noticed that some of the artwork just has the stripe round the bottom which is more easily achieved by drafting a separate piece in the contrast colour. Just don't forget seam allowances! Googling for peplum skirt tutorials will set you on the right track for this.
Your best bet is to experiment with an old sheet (just tack the extension into place until you get the effect you want.