|I have added a picture help with this technique. It shows a flounce made from doughnut already attached to straight piece of fabric, but not turned over. A left-over piece of doughnut is shown as well.
If you are using a wrinkling fabric, it could be quite laborous to press gathered ruffles. It is much easier to press and looks quite different when the ruffle is made of fabric doughnut or a spiral.
How to make:
Say, you are adding a ruffle to skirt hemline.
1) Doughnut. The shape has two radii: inner and outer. Outer radius = Inner radius Ruffle width. The inner radius will determine how full the ruffle will be - from gengle flounce to tight ripples. Experiment on cheap fabric of similar drape to see the results. To calculate how many doughnuts you will need to cut:
# doughnuts = H/[(IR plus SA)*6.28], where
H is the circumference of skirt hemline,
IR is doughnut inner radius and
SA is seam allowance,
6.28 comes from (2 times Pi) to calculate circumference.
I cut the circles on the fold, usually, to make sure they are symmetrical. That's why I work with radii. You can work with a diameter as well, just remember that diameter equals twice the radius (a very painful mistake on math exam back in school)
2) Spiral: instead of cutting the center out of the circles that you cut out for the doughnuts, you can cut it inside as a spiral. The ruffle fullness will change as you go deeper inside the circle. Good for narrow ruffles for neckline (more ruffle in the front) or armcye.