I measure the length of the edge I want to put the fluounce on, then divide that into an equal number (4 or 6). Then I make a curve that long...so,

say the edge of the skirt was 36 inches, I'd divide by 6 and draw a curve 6 inches long.

Then, I put right angled edges on the end of the curve, and measure out however long I want the flounce to be, 3 inches or whatever seems right. Then I draw the bottom of the curve 3 inches below the top curve I made.

Then, I cut 6 of this shape (with seam allowances), sew the edges together, and atatch them to the bottom of the skirt.