This is a sweet and pretty finish to apply to the edges of casual knit garments... and a more neatly-finished-on-the-inside edging, if slightly more involved, than a similar one I wrote previously. This is the edging technique used for this dress.
Firstly; the fabric has to be that sort that when left alone; rolls up at the edges all by itself. Generally, this will only be a reasonably lightweight, single knitted jersey.
Cut a cross-grain strip (that is, with the stretch running along the long edge of the strip) that is as long as the aperture being edged, minus 10% and rounded to the nearest full or half centimetre, for ease of working.
So, for example, if your neckline is 32cm, cut a strip that is 32cm - 3.2cm = 30cm. Or if your armhole is 125cm, cut a strip to be 125cm - 12.5cm = 113cm
The width of my strip here is 4cm (or 1 1/2")
Stitch together the short sides of the strip right sides together, in a 1cm seam. Then pin the right side of the strip to the wrong side of the garment. Your strip is now a bit more than 10% shorter than the armhole/neckline; apply the most stretch when attaching it to the most curved sections of the garment edge, such as the sharpest points of the underarm curve, and the centre front of the neckline.
Stitch in a 1cm (3/8") seam. If the area is not expected to cope with any stretch during wear then a straight stitch is fine; otherwise use a shallow zig-zag, a stretch stitch, or double-stitch with a twin needle.
Trim the seam allowances just a bit, by about 3mm.
Why do we do this, and not just stitch the seam with a narrower seam allowance in the first place? because stitching a narrow seam allowance on a fine flimsy knit is not easy even on the best of sewing machines. Most domestic sewing machines will tend to chew up the edges of a lightweight fabric if you try sewing very close to the edge, meaning you'll end up with an uneven ugly line of stitching. Stitching in a wider seam allowance just means the fabric glides through machine more easily, and stitches up far more smoothly, and then you can trim away the excess width afterwards.
Turn the strip up, and press up...
... then flip the strip over and onto the right side of the garment, encasing the seam allowances and press it in place. Don't worry if you are pressing some of the curl out of the fabric at this point, it will come back!
From the wrong side, pin the strip in place.
With the wrong side of the garment facing up, stitch in the ditch of the seamline between garment and strip. Again, if you are stitching a garment in which has to cope with a bit of stretching then it is probably wise to use a stretch stitch, or a double stitch with a twin needle here....
Give your garment a dunk in a bucket of water and if necessary, if it isn't curling up on its own; ease that curl back into place... it will stay there as it dries, and forever after that. Just don't iron it down!
Voila! cute curly seam, that looks a bit like piping :)
Oh, and that strip joint...? (hehe, strip joint, (smirk)) .... be sure to situate that in an unobtrusive place, like the underarm seam, or just behind the underarm seam if bulk is an issue. (sorry; I forgot to take a picture of this "during") Here the strip seam is about 1cm to the left of the side seam, with the strip seam allowances both pressed to the left and trimmed on the diagonal close to the stitching just prior to the final stitching-in-the-ditch step.