Lay your back body piece out, right side up and identify the notches at the centre top edge. Create your box pleat by meeting the notches together in an outwards direction (the underneath side should have the two edges meeting together).
Staystitch your box pleat down. Ensure that your staystitch seam allowance is less than 1cm so it gets hidden inside the final seam. (A footwidth seam is perfect.)
Lay the two yoke pieces out in front of you, both right sides up.
Lay the back body piece on top of the lower yoke piece, with the right side of the back facing upwards.
Flip the top yoke piece down to sit on top of the back body piece. You should now have all three layers on top of each other with one edge matching. Pin all three layers together.
Stitch all three layers together with a 1cm seam allowance. Back tack both ends.
It should now look something like this. Next you need to understitch the inside yoke - Push both seams upwards but push the outside yoke down out of the way. Turn the shirt over so you are looking at the inside and understitch the yoke seam.
How the understitching from the inside of the shirt will look.
Turn the shirt back over to the right side and press the top yoke up.
Your back is finished and ready to attach to the front.