This is a very nice looking and interesting filling stitch. As the name suggests, the filling would give a fishnet appearance. The fish net is created without touching the background fabric, while pinning down only on the sides. This stitch is ideal for fillings that need a netted appearance.

You need to know the blanket stitch to be able to do this stitch.

fishnet_stitch_1      fishnet_stitch_2
Fig 1: To begin with, make a foundation row of blanket stitch.   Fig 2: When you reach the right end of the row, we get ready to make the fishnet. For that, take the needle in through B and out through C, a point little above B, as shown. The stitch A-B will pin down the last blanket stitch to the side.
fishnet_stitch_3   fishnet_stitch_4
Fig 3: Now, we work a blanket stitch row from right to left, taking in through each blanket stitch of the previous row, as shown. Do not pluck the fabric beneath.
For some, it might be easier to work the balnket stitch upside down than to work from right to left.
  Fig 4: When you reach the end of this row, take the needle in through D and out through E, as shown. This pins down the second row on the side. 
Then, work a row of blanket stitch from left to right. Keep up with this procedure of working blanket stitches back and forth.
fishnet_stitch_5   fishnet_stitch_6
Fig 5: When you reach the last row, finish up by stitching a row of blanket stitch through the fabric, as shown. Through the stitching, you will find the fish net crunching up. Don’t worry, when you finish, the last row of blanket stitch will pull up the fishnet to shape.   Fig 6: The fishnet would look like this.