I pinned that 2cm fold in the centre back together
and sewed the first cm with normal stitching, the main part with wide stitching
and at the other end again a few cm with normal stitches
then I ironed the fold to one side
I will unpick the wide stitching after finishing the jacket, to give the back a little more room to move
next I pinned the side parts to the back part
and the front parts to the side parts
and the shoulder seams, sewed all and ironed the seam allowances apart
then I pinned the sleeve into the armholes and sewed and the lining is finished, next week I will work on getting the lining into the jacket.
I hope this was helpful for you, do not hesitate to ask, if you have any questions!
keep sewing and smiling