|I've seen a number of articles about sewing buttons recently, and they've all been missing some information. I think it's important to know why a button needs a shank, whether the shank is part of the button, or made from thread. It's also important to understand just how long the shank needs to be.
The shank leaves room underneath of the button, for the buttonhole side of the fabric, when the garment is buttoned. The length of the shank needs to be as long, or a tiny bit longer, than the buttonhole is thick. If the shank isn't long enough, the fabric will pucker and look bad [squished] when the garment is buttoned.
A sheer buttoned blouse is very thin. A lined and underlined winter coat is thick. A treatment like a bound buttonhole is thicker too. To determine just how thick the buttonhole side of the fabric is, you need to measure. One way that I've devised to measure this thickness, is with a strip of paper. Click here for 4 photos in 1, showing how to do this with your 'test buttonhole'.
Once you know how thick the buttonhole side is, a regular button [without a shank of its own] needs to be sewn on, leaving a thread shank that long. I have used a pin, a toothpick, or a piece of wooden dowel [1/8", 3/16", etc.] as a spacer, to form the thread shank. The pin is usually enough for a thin blouse or shirt...the toothpick is more for a medium-weight blazer or jacket...the 1/8" dowel for heavier fabrics, bound buttonholes, and 3/16" or bigger for really thick winter coats.
You sew over the spacer by hand, as shown here.
Or you can sew over the spacer by machine, as shown here. I am just holding the toothpick in place. Be sure to turn the wheel by hand a couple times to check that your zigzag is set to the correct width for the button's holes. This is sewn with the feeddogs down, or with a stitch length of zero.
A longer shank [longer than toothpick width] is easiest sewn by hand [doubled, knotted thread], so the loose threads can be reinforced [you can sew by machine, but you'll need to leave long thread tails, and still use a hand needle for the reinforcing step]. Once you've sewn through the holes of the button 5 or 6 times, remove the spacer. Take your needle and thread back up through the fabric, but not through the button. You'll now work between the button and the fabric...
Pull the button away from the fabric, then wind your threads around the loose, previously sewn threads a few times, pulling so it's a snug wrap. Then stick your needle through the wrap until your thread has a loop. Put the needle through the loop, to form a knot. Wind a few more times, knot again. Keep winding until the shank is covered, and knot twice. Fish your needle back through the layers of fabric a little ways, then pull a little and clip the thread close to the fabric [your thread end will disappear between the layers]. This makes your thread shank sturdier, so the button doesn't just flop around, and it can take more wear and tear.
Other Button Tips
On a woman's blouse, jacket, or coat, a button should always be placed at the fullest part of the bust. If you don't do this, the garment will have an odd gap there, even if the garment has plenty of ease. Button placement for the rest of the garment should then be spaced, measuring up and down from the bust button. Patterns usually show button and buttonhole placement, but this is just a suggestion. Your changed placement may require slightly different spacing than the pattern suggests, or an extra button or two.
For machine sewing, a washable glue stick is handy to use for holding the button in position temporarily.
Patterns usually suggest a button size. This is important, if the blouse or shirt has a placket...the size suggested will be in proportion to the width of the placket.