Basically you want to do the same method that we did for the stripes on Super Moon’s skirt. The white piece is a really long zig zag that has to be drafted along with the blue of the skirt to make it all lay flat.
With Sayaka all you need to measure is how big you waist is, and how long you want the two side seams to be. First divide your waist measurement by two, then divide that new number by 5, that will give you your pleat measurement for how wide you need all of them to be since Sayaka has 5 box pleats on both the front and back of her skirt. (and you are drafting both the front and back at once) And then finally divide your last number by 2 for how wide the insides of your pleats are going to be.
So if I was making the skirt for me:
my waist: 32 inches
shorter side: 11 inches
longer side:14 inches
my waist divided by 2: 16 inches
the 16 divided by 5: 3.2 inches
the 3.2 divided by 2: 1.6 inches
I’ll just use these measurements while explaining, please replace them with your own, unless your waist is the same size as mine, in which case, party.
Now get you self a long, long, long piece of paper that is as at least wide as your longest side measurement, a ruler, and a pencil. The easiest way i have found to draft box pleats is start with your smallest measurement and alternate between that and the second smallest. So you would measure off the 1.6, 3.2, 1.6, 3.2 etc. Sayaka’s skirt is all box pleats so you can hide your side seams completely in it. So to start marking off your pleating lines, you are going to want to start with measuring off your smallest measurement twice before the larger one, so 1.6, 1.6, 3.2, 1.6, 3.2, 1.6 etc. The reason you are doing this is the side seams will be right in the middle of the insides of your side pleats so you need to cut it in half. Then continue along till you have all the pleats, it will be a VERY long piece.
Here is where all the math you just did pays off. Take your paper and start folding the pleats into the paper starting with the inside of the pleat on the side. (much like the side seams of our sailor skirts) Once you have it all pleated into box pleats, you want to iron the paper on a low heat, or lightly tape the pleats closed with scotch tape to make sure it does not move or open on you for the next two steps.
On the sides of all the pleats mark off your 11 inches on one side, then the 14 on the other, then draw a diagonal line connecting the two. This is now the bottom of skirt. Form there decide how wide you want your white band, (i think ours was an inch) and draw another diagonal line on top of your first, and this is now your stripe on the bottom.
Now do not take the pleats apart yet, and literally cut along the bottom with all the pleated layers together to get the diagonal hem of the skirt, and repeat for your white stripe.
Then open it up to reveal your perfectly matching white stripe and skirt pattern!
Add seam allowance and you are ready to go. Cut two of the white stripe so you can hem it with itself, and then attach to the blue part.