ok, so I took all the suggestions, and tried to cut what I wanted x 1.5 where I needed to shirr.
I used my longest straight stitch (5.0, on my Bernina 730, but not sure exactly what 5.0 means. it might be millimeters of length?), I handwound my bobbin with elastic thread. (OH, if anyone else has a Bernina or an industrial, whatever, where you put your bobbin in a bobbin case, and THEN put it in your machine, you might need to loosen the screw on the case. At least, I did. when I just put the elastic thread bobbin in the case without loosening the tension on the case, the elastic broke right away, repeatedly, but as soon as I loosened it a bit, it went through just right. I think if I start shirring a lot, I'll invest in another bobbin case, cause it got annoying tightening and untightening the case as I switched back and forth between sewing and shirring)
This is my first attempt (luckily out of scrap fabric).
huge (bad maternity-esque?), and, while theoretically covering me, there was SO much stretch that the teeny tiniest tug from an 8.5month old, and I'd be the flasher mom.
so I re-cut it today (sorry I was AWOL yesterday, I got a migraine, and as a nursing mom, couldn't take anything for it!)and got THIS!
wrong bra and skirt, obviously, and too short for my taste (I'm thinking of adding a ruffle to the bottom, cause I really do like this fabric), but this was how long my scrap was, so...
It's only about 3 inches wider than I would have needed, unshirred, so that comes out to like x 1.1, instead of 1.5, but I really like it.
I think I might try it in super soft jersey next, (leftover from THIS dress).
As much as I LOVE this scrap (thanks Grandma!), I don't have any summer bottoms to go with it. Clearly I need to do more sewing.
BTW, the winner of the giveaway is commentor #1, Tien, so send me an email (lafamiliaburke (at) gmail (dot) com) with your address so I can send you your crochet thread!
thank you all so much for your advice!!!