Cut out your garment, two fronts, a back and two yoke pieces.
You can see I’ve used a fabric for this demonstration with a definite right and wrong side to make it easier to see what is going on.
Working with the back panel and the two yoke pieces, sandwich the top edge of the back between the yokes making sure that when everything is turned the right way around both right sides of the yokes will be facing outwards, away from one another. Sew along this edge.
As in the picture spread both yokes out, right sides uppermost and roll the back panel up towards the centre.
Position the front pieces, as they will eventually appear, on the fronts of the yoke. It is important to check that you have both of the shirt centres in the correct place, unpicking is tiresome.
Roll up the two front pieces and place on the seam edges with the right sides of the fabric facing one another.
Fold the second yoke piece onto the first, sandwiching the rolled up back and fronts inside the yokes. Pin the front seam edges and sew through the three layers on both fronts.
Pass all of the rolled up pieces through the open neck edge and open out. Press the seams flat and topstitch 1/8th inch away from the seams to create a nice finish.