a round object with the circumference the same length or large than the edge of the garment you need to attach the flounce to
a round object that is smaller than the object above (when picking these objects, bare in mind that the distance between the smaller circle and the larger circle is be the height of the ruffle)
You can use a full flounce pattern or a half pattern as I did. The reason I wanted to make a half circle flounce is that I needed two layers of flounce. I added one to the front of a blouse and the second one to the back of my garment, then I joined the two piece of the blouse together at the sides.
The process started with measuring the bottom of my blouse and divided it in half.
I used this measurement to draw a half circle on a large piece of paper, using a super large bowl.
After deciding how wide I wanted the flounce, I added that measurement to the circle I drew above and made a smaller circle, inside the larger one, using a smaller bowl.
Then I used the half circle to cut two flounce ruffles from my fabric.
You can also make a half circle, fold the fabric in half, place the half circle on the fold then mark and cut out. This will give you a continuous circle that you need to cut open so you can attach it to the garment.
Making a full flounce pattern
Another alternative is to make a full circle directly on the fabric.
Lay your fabric square on a table and place the larger round object on the fabric.
Using a fabric pen, draw a circle around the object, onto your fabric.
Get the smaller round object and draw a smaller circle inside the larger circle.
Using your scissors, cut out the circle in the fabric.
Attach the inner circle to the edge of the garment, using a regular stitch then finish the seam with a zig zag stitch or by serging it.
Finish the outer circle with a regular hem (using this tutorial for sewing a round hem), a narrow hem or a rolled hem, depending on the type of fabric you're using.
Please note that if you're using striped fabric, the direction of the strips in your flounce hem will change in all kinds of different directions, giving an unpleasant look to your ruffle. So you might want to use plain fabric to make your flounce ruffle.