In my experience it’s the way that random lines of stitching show that announces, “…and the groom’s sister made the flower girls’ dresses…”
For example, when you’re required to do stay-stitching, say on a collar facing and the neckline, and then when you attach the collar, you don’t get the collar and neckline seams matched precisely, so you have bits of the stay-stitching peeking out. The way around this is to pin, and sew, the collar assembly together with insane patience.
Same thing for sewing the sleeve in the dress–do it very, very slowly, to make sure there’s no line of basting stitches or stay-stitching showing.
And the zipper of course has to be put in with the speed of glaciers flowing. I found that it helps not to sew over pins that are holding the zipper in place, because it leads to wiggly detours in the line of stitching–do like the books tell ya, and baste that sucker in there with obsessive attention to detail, and then sew it in there likewise, with all pins removed.
Any buttonholes need to be done with a buttonholer, not with the old zigzag-and-reverse-stitch technique. Borrow a buttonholer if your machine doesn’t have one.
Reverse stitching to lock in the stitching needs to be done with parsimony. If you’re in the habit of running the machine backwards and forwards “a whole bunch”, like 10 or 20 stitches for that, it’s going to create bulges in the line of stitching when you try to iron the seams. Be sparing with the backstitch, and just use a couple stitches and then stop. The dress won’t come unsewed.
Iron all seams as you go according to the pattern’s instructions (if you didn’t already know that). Flattening a sewed-together component makes it easier to handle for the next step, it’s less poofy, and makes the pieces fall into place the way they’re designed to.
Sashes. Sashes are EVIL. There’s a federal mandate that flower girl dresses must have sashes, however, so I think you’re for it. When you turn them right side out, after having sewed the right sides together, and you press them flat, if your line of stitching wasn’t absolutely ramrod straight, you’ll have a sash with weird wiggly bits where the line of stitching curved slightly. The way to avoid this, again, is to sew with maniacal patience.
Hems. Hems are second only to sashes in terms of evilness. You MUST pin the ENTIRE hem before you try to sew it. Don’t even think about pinning it every couple inches or so, and then trying to ease in the fullness as you hem it. Pin, pin, pin. You’ll need to buy more pins, trust me.
And measure, measure, measure. Get yourself a hem gauge. This little puppy will be worth its weight in gold by the time you’re done. Use it ruthlessly, don’t rely on dead reckoning or guessing as to where to put the next pin. Measure obsessively.
And: have her try it on, AFTER it’s pinned and BEFORE it’s hemmed, so you can be 100% sure it’s going to look right. Generally flower girl dresses need to be hemmed on the long side rather than on the “above the knee” short side, so before you undertake the immense task of hemming, not one but two flower girl dresses with those full skirts, you’ll need to be sure you didn’t measure wrong on a wiggly little girl so her dress ends up looking like Shirley Temple’s with her knobby little knees hangin’ out.
Sew lace hem tape on the hem before you pin it. It keeps the rough edge of the fabric from unraveling, makes the hem edge easier to handle while you’re patiently working your way around those miles of hem fabric with the hem gauge, and it looks pretty, too.
Do NOT assume, “Well, I cut the fabric exactly according to the pattern, so I don’t have to actually measure the hem all the way around, I can just assume that at the side seams it’s 4” of hem allowed", and then just pin it up 4" all the way around. Trust me, you didn’t cut the fabric exactly according to the pattern, and you do have extra fabric in some parts of hem, and less in other parts. This is a good way to get that touchingly homemade droopy-skirt look.
Use embroidery floss in a complementary color to make sash holders at each side of the girl’s waist.
Finally, allow plenty of time for this project. It’s not going to be something you’re going to be able to whip up in a weekend, or even in a week. I’d allow at least two weeks per dress. Seriously. The wedding’s in June? Start now. Prevail upon that future SIL to make her selection by March 1 at the latest.
And remember the girls may have increased a half-dress size between the time the SIL picks out the pattern and the time they actually have to wear the dress, so when you’re choosing a size to sew in March, go with “tiny bit too big right now” rather than “fits perfectly right now”, because what fits perfectly in March may be a tiny bit too small in June.
I’m assuming you don’t have to worry about the possibility of one of the flower girls coming up preg the week before the wedding? 