Insert your crochet hook through the first hole from front to back.
Insert your crochet hook through the stitch that you will knit next.
Carefully pull that stitch off your knitting needle and through the first hole of the button.
Drop the stitch and insert your crochet hook into the second hole of the button from back to front.
Catch the stitch with your crochet hook and pull it through the second hole of the button.
Place the stitch back on the knitting needle and knit as usual. The stitch may be tight – you can knit through the back of the stitch if that is easier.