It is a variation on the one row buttonhole (though there is a little bit to do on the next row to finish it). My problem with the usual way of doing it is that the cast on part looks different to the cast off part.
Here are the instructions if you would like to try it.
The cast off part is exactly the same as the regular one row buttonhole:
Row 1: Knit to the point where the buttonhole will be placed.
Slip the first stitch on the left needle as if knit.
Move the yarn to the front of the work and leave it there. Slip the next stitch from the left needle to the right needle as if to knit. Pass the first slipped stitch over the second slipped stitch and off the needle. Repeat until you have bound off the number of required stitches for the buttonhole.
Slip the last stitch from the right needle back to the left needle. Turn. Bring the yarn to the front of the work.
The cast on part uses the crochet cast on, so you will need a crochet hook to continue - also a stitch marker would be handy.
If you have followed the above instructions so far (and you are using stockinette st) you will have the purl side facing you.
Look at the first stitch on the left needle and count two bumps down. Pick up the bump with the right needle and place it on the left needle oriented with the right "leg" towards the front. Purl this stitch and then place it on the crochet hook.
Crochet cast on 1 stitch - good instructions for that here - Crochet Cast On and then place the stitch marker on the left needle.
Crochet cast on the same number of stitches you cast off pulling the edge chain fairly tight as you go to produce a nice edge.
You should have one extra stitch overall - plus the loop on the crochet hook.
Turn the work again to have the right side facing, slip the loop from the crochet hook onto the right needle. Slip the first stitch from the left needle onto the left needle and pass the crochet hook loop over it..Return the first stitch to the left needle, pull the working yarn to tighten that loop a bit and then knit to the end of the row.
Row 2: Purl to the marker. Remove the marker, and then slip the next two stitches onto the right needle as if to knit, slip them back on the left needle and then purl them together through the back of the loops. (This loses the extra stitch by hiding it behind the next stitch)
Purl to the end of the row.
And you are done.
I tried it without the extra stitch and decrease on the second row, but I don't think it gives as strong and as nice a finish.
I would love to hear what you think of this and any suggestions for improvements.