I'd probably take triangle shaped skirt pieces that are cut longer than needed and gather the sides, and then sew them to each other. It looks like there is another line sewn over the seam or ribbon/trim sewn over the seam, but couldn't zoom in close enough on the photo to see for sure.

Gored or A-line skirt with vertical ruching- you can either stretch some elastic that is the finished length and sew it to the full length of the skirt (how they did it, probably) along each gore seam (or, for A-line skirt, along side seams plus evenly spaced divisions, however many you want). Or you can sew a casing out of skirt fabric or wide flat bias tape, sew to the seams plus a line of stitching down the center of each casing, and string through with ribbon or cord if you like the look of dangling ends.

If you want one in a white or off-white fabric, you can use drapery tape, which already has the casing and cord, but it only comes in polyester these days, so you can't dye it.

That's cute. It reminds me of the old white curtains that were popular during the 60's and 70's.

Cool!

Azeeza

It's an ordinary gored skirt, cut very long, and gathered up at the seams between gores.

Pick any gored skirt pattern you like. If necessary, adjust the length so that the hem falls where you want it.
Measure the length from waistband to hem. Multiply that by 1.5, that's your cut length.
Now, extend the pattern to the cut length.
Assemble the skirt, and gather it up along the seams. I suggest using the casing method that Margaret suggested, because that allows you to adjust the gathers and fullness for the best effect.

The finished look depends on the fabric hand. Fabrics that are too soft and heavy might just collapse at the bottom, sagging instead of ballooning. Crisp or stiff fabrics will be very puffy and balloon-y

Buried in the depths of my closet is a bridesmaid dress, about 20 years old, with this style skirt!

I've experimented a bit with this- the simplest way is to sew each gore with elastic. Stretch it as it goes into the machine and it will ruch up all on its own.
I second the 'use stiff fabric' point. That's very important for the final product to look poofy instead of just draping.

Hey, there!

So, the way I see these skirts (and reading the descriptions) is shirred. Basically, you have long panels that you gather into the seams you use to sew them together. It's like a sideways tiered skirt, yes, but instead of attaching a gathered panel to the straight bottom of the previous panel, you're going to gather both sides and sew them together. Does that make sense?