Preparing work is the same as in Ways to Work Mitered Corners (1) explained. For the sake of simplicity I repeat it here.
It is important that the hem width (the spacedistance between the withdrawn thread line for the fold and this the withdrawn thread line for securing the hem securing) is the same on both sides of the corner.
Iron your piece and lay it plain flat – right side face down.
Following the thread of the withdrawn thread line for securing the hem securing up to the edgewhere it meets the withdrawn thread line for the fold, mark the point exactlyctly the point. Do the same on the adjacent side of the corner.
Using a #2 pencil draw a diagonal line to connect both marks.
Trim the seam allowance.
Fold the seam allowance.
(One can mark the pencil line using short Running stitches to ease exact folding.)
Fold the fold.
Lay the hem in place and secure it.
Hand sew the corner closed using Blind stitches.
Both methods, done carefully, will produce perfect corners – it makes no difference whether Antique Hems stitches,
or Peaholes are used.