I started with this mid-calf- length skirt that I got years ago and no longer wear (the truth is that I still like it but just because I can still get it on doesn't mean it "fits").
I don't remember exactly what it was made out of, but it was quite stretchy and it had an elastic waist. I laid the skirt out flat, and found a cotton shirt that I wanted to use as a rough guide in making pattern pieces for the maternity shirt.
I pinned the edge of the sleeve to the top of the skirt, so that the elastic waistband would be the new sleeve edge (thinking ahead to save as many steps as possible- no hemming required here!)
I did the same for the other sleeve, cutting generously to include a seam allowance. At this point I also cut off the tags and some little belt loops that were attached to the waistband. I folded the sleeves exactly like they are in the picture, but with right sides facing (inside out) and I sewed along the short edge so that the sleeve was then round, like... a sleeve.
Then, I pinned the shirt on the the skirt so I would have a cutting guide for the bust of the shirt. I picked a bigger shirt because its always good to make maternity clothes a little bit larger than you would normally wear (or you could actually use a maternity shirt as a guide if you had one handy).
After the shirt was pinned, I cut a similar outline from the skirt, and continued cutting down to the bottom of the skirt, allowing it to flare slightly (I actually went back later after the shirt was made and sewed it so there was less of a flare because I didn't like the fit).
On the shirt front, I cut a deeper neckline than the back.
Here is another attempt to show the detail on the front of the shirt. There are 5 ruffles of differing lengths, sewed on in a somewhat orderly manner, but nothing too mathematical!
And that's it! Embellish and tailor to fit your preferences and your body type, and you have a brand new shirt that didn't cost a thing to make!
I didn't have any extra fabric left over- I ended up using every bit of it! Here I am, at 6 months pregnant.
Note: I opted for a basic how-to rather than a detailed, step by step tutorial because, quite frankly, I forget to take pictures of every step. However, if there is an interest, I can certainly add more detail- and please feel free to email or comment if you have questions, and I'll do my best to answer them!