Knit until you reach the point where you want your button hole to be.

Knit the next 2 stitches together.

Bring your yarn to the front of your work, by bringing it over your right needle.

Now return your yarn to the back of your yarn by bringing it through your two needles. This will have formed a new loop on your right needle.

Knit your next stitch like normal. This will have formed your button hole, on your next row it will become more clear.

Knit to the end of your row.

Purl your next row. Now you can properly see where you made your button hole.