It is best to try creating flounces with scraps of fabric until you feel comfortable with the process.
Trial and error have shown me that a flounce is best if the lower end is wider than the top end. (in the photos, the top is on the right and the bottom is on the left.)
A flounce can be made from an oval or a round piece of fabric. You can start with a square, but you will have to cut the corners off.
Cut a circle near one end of the fabric piece as I have done in the photo.
Straighten the flounce to see if it is long enough and wide enough (don’t forget to include seam allowances).
Trim the shape or use a new piece of fabric and start over until you are happy with the flounce.
When it is ‘perfect’, transfer the shape to pattern paper and note any grain lines and instructions on the pattern piece.