The easy way to approach this is to hem each panel separately first, then sew the seams of the pleat. This has a disadvantage in that the seam allowance is readily seen at the hem fold. Instead you can try the method I’ve used here.
Sew the panel seams. I have marked the hem foldline with a small clip, and above this cut a V notch from the seam allowance exactly where the overlocked hem edge will fold to. For instance, if your hem is 4cm, then the V notch will be positioned at 8cm. Cut the V notch to within 1-2mm of your stitching line:
Press the seam within the hem allowance open:
One last thing is to fold the inner fold of the pleat into it’s finished position, then on the inside, edgestitch where the hem folds:
This helps hold that foldline in position forever, and makes it easier when pressing too!
A box pleat is the reverse of an inverted pleat, and any seams on the inner fold will lay towards the centre. You can use this hem technique on a box pleat as well, or any other pleat designed with a seam – it gives a tidier finish, don’t you think?