Pattern pieces five and six form each sleeve. Once sewn together, piece five will be at the front and six joins the back
I suggest first lining up the underarm seams of one sleeve and the body part and stitching within the seam allowance to hold it in place. Once I had done this I lined the bias tape on the sleeve with the neck edge of the body, at the front and back and also held each in place with a couple of stitches.
I pinned the front's together and they lined up fine, on the back there seemed to be much more fabric on the body part than there is on the sleeve part, I started at the neck and pulled the end of the bias tape down towards the underarm before pinning, I carried on pulling the back of the sleeve as I pinned it and eventually I had the sleeve fabric sitting evenly with the body.
I basted in place first and checked there were no gathers or tucks, then stitched. Once sewn you would never no the pieces had not seemed the right lengths. I guess this is some clever design feature, I have no idea why or how it works but fully trust Lisette that it is no mistake.