1.  Mark your final edge.  I use 1 1/4" for rug warp when I'm doing a whipped edge.  Use a pencil and drag it in the ditch.  Use the method described in Basic Techniques: Drawing Straight Lines.
2. Zig zag around your piece with the presser foot touching your loops.

3.  Run another row about ¼” from the first as shown.  Be sure to stay inside your final edge. 

4.  Stitch a row of straight stitching over each row of zig zag.

5.  Now cut off the excess at the pencil line.  Zig zag the raw edge to prevent fraying.

6.  Fold the corner in so that the tip of the corner just touches your hooking.  The sides of the triangle should be in line with the rows of your hooking.  Mark the 45 degree angle made by the fold using a pencil or marker along the edge of the fold. 

7.  Unfold.  Working from the wrong side so that you can see your mark, zig zag just inside the line.  Cut on the line being sure not to cut through the threads of your zig zag.  Repeat for all corners. 

Be sure to stitch the corner diagonally BEFORE cutting.  If you cut first, the corner will stretch out of shape because you are stitching on the bias.