Detached buttonhole stitch / net stitch is a filling stitch that is stitched on a base of a chain stitch / stem stitch / back stitch. This buttonhole stitch variation is worked on the surface of the fabric (needle does not poke fabric) and is anchored to the fabric with the help of the base stitches.
Make a border with chain stitch.
Bring the needle up in a corner and stitch a buttonhole stitch like shown below. Needle does not poke the fabric.
When a row is completed, anchor it to the base stitch by passing needle through a chain loop.
Continue the next row. The second row is built upon the first row.
The last row is built on the previous row but it is also anchored to the base chain at the same time.