Sewing up the pocket is very straightforward
Begin by creating the Inverted box pleat.
To create the box pleat begin by folding the pocket piece in half with the right side on the inside. Mark a line 5cm in from the fold, this is the pleat line. Mark again 9cm down from the top edge and 5cm up from the bottom.
Sew along the pleat line to the marked points leaving a gap in the middle. Open out the pocket and press the box pleat centrally over the pleat line.
Fold under 1cm across the top edge of the pocket, then fold the top edge under again 3.5cm, this creates the pocket facing. Topstitch along the bottom of the facing to secure it in place.
Press under the 1cm seam allowance along the three remaining sides of the pocket. This will act as a guide later.
Fold the pocket with the right sides together to pinch together the bottom corners. Sew across the corners. Trim off the excess fabric from the corners and turn the right way round.
Fold over the sides and base of the pocket so they are in line with the pocket corners. Press to create a crease that runs around the front of the pocket. This creates the boxy shape of the pocket.
Edge stitch along each of these creases, stopping at the corners and starting again once the corner has been turned.
Now you’re ready to attach the pocket to the main fabric.