In this sample hem stitch has been worked to gather the border threads in bunches of two.

Work your chosen stitch down the length of the vertical border and with the same thread, begin the corner stitch. Bring the needle up horizontally into the last bar of the previously worked section.
Take the needle down into the corner, catching the loop.

Bring the needle up two threads into the fabric.

Take the needle down into the corner, catching the loop.

Bring the needle up two threads into the fabric.

Take the needle down into the corner, catching the loop.

Bring the needle up in the corner through the first loop.

To finish off neatly, take the needle down two bunches into the vertical border and run the needle through the stitching so that you can cast off on the edge.

The finished buttonhole corner.