I love buttons and have been incorporating them into my designs as often as I can. Whether functional or decorative, you want to make sure they're going to stay in place and, if necessary, stand up through washing and ordinary wear. I've put together some tips and techniques for sewing buttons on knit and crochet pieces.
I love chunky and bulky yarns and they provide a fabric with which you can use some bold and beautiful, large buttons.
With some yarns, you can adjust the gauge of crochet stitches, or use longer stitches to accommodate the size of the buttons and just button anywhere in the stitches, without having to create an exact button hole. For example, with Wool Ease Thick & Quick yarn, a size P crochet hook, and a double crochet stitch pattern, a 2" button will fit nicely between the stitches. For an extra large button, a triple stitch might fit better. Test your stitch size with what buttons you want to use, but it's best not to use too small of a button if you use this method. You don't want it slipping off all the time. Also, smaller and 4-ply yarns might split and get caught on the buttons, that's why I tend to do this only with the thicker, bulkier yarns.
If you can, plan on using the tail of the yarn to sew on the button. If the tail yarn is near where you want the button to be, the holes of the button are large enough for the yarn and needle, and the yarn is appropriate for sewing on a button, just weave it through a few stitches and sew the button. Knot the yarn and weave in the ends. You'll need a longer yarn tail if you choose to do it that way. Most types of yarn would be appropriate for sewing with, but be careful if you're using single ply wool or loosely spun fibers since you can't pull snugly, they might pull apart. I do this a lot with some of my button wraps. The corner button uses the yarn tail, but I'll sew with an additional piece of yarn when adding a second button in another location.
highly recommend having a sewing kit on-hand. Often times, you'll need a
particular color of thread to sew on a button on knit or crochet
fabrics. Instead of buying a whole spool of thread in a color you may
not need again, you could probably find a close enough color in a sewing
If you're using sewing thread to sew on a button, it's best to double the thread and knot it. If you only knot one end of the thread, leaving a loose tail, that
knot might slip through the yarn strands and not be a secure start to
sewing on the button. It might work for some fabrics, but not when
working with yarn. I like to start by working the needle with knotted double thread around the yarn, pull the needle through the doubled thread and pull tightly. This makes a nice foundation to start sewing on the button
One last tip, in regard to button holes, is to whip stitch on the side of the button hole. As you can see here, a button hole was created by skipping a stitch. That skipped stitch might be loose enough to catch on a button, so I like to whip stitch a few stitches over it to tighten it up and create a more prominent button hole. If you happen to have a yarn tail to weave in near the button hole, whip stitch around a loose stitch if desired.
For more button tips see this great post at Crochet Me.