Most of the work is on the inside of the garment.
FRONT AND BACK LINING PIECES SEWN AT SIDE SEAMS
INNER SHELL OF WAISTCOAT
Pin the two sections, right sides together. Make sure the armhole and front edge pieces are in line. In this example, I had to let out one of the side lining seams because it was too small. It fit fine after this adjustment.
GAP IN THE BACK
Sew the armholes, neckline, front and lower seams together but do not stitch the shoulder seams yet. Also, make sure you leave a gap of 15cm in the centre lining piece to allow the garment to be turned inside out. Before reversing the waistcoat, I pressed the seams together after they were stitched. I also clipped bulky seam areas and notched the curves. Turn inside out and press.
Try on the waistcoat and pin the shoulder seams together to get the new seam allowance for that area. It should be 1.5cm but is liable to change if you have a different body shape to the pattern standard. Turning in the inner shoulder lining seams by hand is the easiest way to finish this area. Then you must sew an invisible stitch along the centre seam lining (the gap).
And then your waistcoat is finished and all you need to do is put on its fastening!
In the below example, I used one hook and eye to close the waistcoat and sewed cord onto the outer edge for texture and decoration.