Knit in stocking stitch all the way until you reach the point you want your button hole to be.

To knit the cast off button hole, begin by knitting 2 stitches in order to cast them off, cast off the first one by taking the second stitch over the first one.

Do this until you have cast off 3 stitches. You will now see a hole that forms the bottom half of the button hole.

Knit to the end of the row as normal.

Because you are working in stocking stitch, you will now begin working a purl row until you reach the edge of the button hole.

Turn your work and cast back on the 3 stitches that you cast off before. Do so by inserting your right needle into the first stitch on your left needle, bring your yarn around, pull it out and place it back on the needle.

Once you have cast on 3 stitches, turn your knitting over again and continue with your purl stitch all the way to the end. You have now completed your cast off button hole.