|ETA- I now see that StacyCK already has a great tip on this..(oops! I'm going to leave mine for the photos, in case anyone needs the visual.)
When it's Saturday at my house that means 1 1/2 hours of quality sewing time while the girls watch Saturday morning television. This week, it was entirely spent mending 4 sets pajamas, which made me remember that I wanted to make a tutorial on How to Shir.
My youngest has a nightgown from Gymbo which the shirring has disintegrated from overuse, but the thing is still one of her favorites.
To shir a new garment, first you would test your fabric by measuring a small piece (use at least 8 inches to get a good test), shirring it, then using your steam iron to shrink the shirring. You can estimate about 2:1, but for a more fitted garment, I like to run a test. Then remeasure the piece to determine how much shirring will shrink the material. (Similar to tight gathering.) For example, does it lessen the length by half? then you need to double the fabric width for that section.
In this case, it was a nightgown already being worn, so I'm just winging it.
The first step is to wind elastic sewing thread (this is usually in the notions/elastic section and not in the thread section of your fabric store) by hand around a bobbin. Do not pull the elastic tightly, just firmly. Do not try to do this on your machines bobbin winder as it will end up in kicking a wall, or speaking like a sailor.
Then place your bobbin in your machine as normal. Us a slightly longer stitch (I like to use 3.0) on your machine. I do not adjust the tension on my machine, though you can play with this adjustment if you are using a decorative thread on top. (I used just different colored threads for this project.)
When shirring on a new project, shir prior to sewing side seams or bindings. This way, the end strings will be caught in the seam. I try to use even numbers of rows whenever possible- then tie the ends together in a knot. In this case, rather than rip out the bindings on the arms, I tacked the beginning stitches, then trimmed close to the tacking when I was done. This was done using the tacking button on my machine.
If you do not have a tacking button, stitch at 0 stitch length for a few stitches with a short zig zag. I do not recommend backstitching with elastic thread for tacking.
Sew the rows fairly close together- here this was predetermined for me, but on a new project I would sew them no more than 1/2" apart for a yoke, possibly larger for a back of a dress. Here the previous row of stitching just lines up with the outside of my presser foot.
Please note that it will look like your gathers are not gathering "enough". The elastic shirring will shrink up after you steam iron it. In the next photo, I have completely gathered, but only steamed one section. I do this on cotton setting with full steam. Here's a photo of the yoke gathered, but not steamed-
Here, the portion on the left has been steamed, the portion on the right has not. See? It really does make a difference. The photo just below it is a comparison of the nightgown prior to shirring.