After preparing the edge, fold the backing edge toward the front with the raw edge touching the last row of loops.  Loosely baste in place.

When you get to the corner, continue stitching to the next side with a stitch or two then backup to the corner.  Stitch the edges of the mitered corner together to make a sharp corner.  This will make the next fold easier and neater.

Continue around your piece until complete.

Next turn the edges forward again so that the folded edge touches the final row of loops.  Baste in place.

When you get to the corner, stitch even with the last loop.  Fold the next side into place and take a few stitches at the base of the corner where the sides meet..  The rest of the corner will stick out, but ignore that for now.

Now press the corner towards the finished piece with your finger.  Take several stitches along the sides of the package to hold it in place.  See the diagram below.

Don't skimp on this step.  The stitches you make will help to make a smooth corner when you whip the edges with the yarn.  This thread "bridge" will allow the whipping yarn to move smoothly up and over the corner.