My son was home for the weekend so he was able to try on the muslin. There's lots that needs to be changed. Mostly on the back though and I'll have to think about how to do it.
I should have finished my notes as I was doing the sections because I'm not sure if I remember everything now. Mike Maldonado has a video tutural for one way of doing flat-felled seams. Instead of putting the raw edges together, sewing the seam allowance, triming one side and then folding the wider edge over and under the trimmed one before edge stitching, he shows a quicker method. He starts by leaving a gap between the edges. Then sews the seam allowance and doesn't have to trim back one of the edges because it is already shorter than the other side. The back of my muslin is much wider than the front. As I wanted to try method I used the back for the wider side and with right sides together made the difference between them 6mm.
I wanted the seam to be facing the same way on both sides. So for one sleeve I had to start from the hem and sew to the cuff.
And for the other the cuff to the hem. I was sure that this seam was going to be difficult but once I had the shirt postioned correctly the sewing was no problem.