Here's how she describes it
One more detail to consider is yardage. If you're not using a pattern that indicates the yardage needed, you'll have to calculate how much additional fabric you'll need. You have to add the widths of the folds and the distance between them to your overall cut measurement to make sure you have enough fabric to pleat.
So heres an example, if your project is let's say a skirt, then you need to know your finished waist size (28 inches) so you'd need maybe 30inches if you have a zipper. Then you decide how many pleats you want, is it around the entire waist? Or maybe just four in the front. Let's say you pick 4 pleats in the front. Now pick the width of each pleat, 2 inches each. So 2inch X 4Pleats = 8inches extra you will need to add to your width. So 30 inches + 8 inches for pleats= 38 total.
Does that make sense? Try just pinning pleats on a fabric scrap the size you'd like for your project, then mark the folds, unpin and measure from pin to pin, that will give you the width of each pleat.