It's very straight forward, though I don't have a step by step tutorial. It is basically a combination of my Flamenco dress and a simple knit skirt with fold-over waistband.
I measured, cut, pinned and sewed two rectangles together to make the skirt. I cut off the shirt at about my belly button. I decided to use the bottom part of the shirt for the waistband, since I thought that would be much more comfortable than elastic on an expanding belly. I also thought I would need a way to make the waistline adjustable so she could wear it longer. I decided to make two casings and then top stitch them. I cut the hem off the bottom of the shirt and I was left with a tube of knit. I first took the sides in just a bit (because the bottom of a shirt is made to fit near your hips, not your waist, right?) and as I was doing so I sewed one top stitched casing to each side seam, to make a tie for the back of the dress.
I made sure to sew the casings to the bottom part of the knit tube.
Then I folded it in half. I sewed the shirt to inside of the knit tube (just below the fold) and the skirt to the inside of the bottom part (the bottom part of the tube was the end of the fold that was open and I sandwiched the skirt between the two pieces.) The waistband runs all around the dress and the ties on the side cinch the dress in and tie in the back.
And now I kind of want one. I mean, not that I'm pregnant. I'm not. But, say I got pregnant sometime? I'd make me one of these.
In the meantime, maybe I'll just put some pleats on a t-shirt for me. Yeah. I could do that.