OK. So I have finished the lower armhole and joined the yokes to the rest of the tunic. I have also sewn the shoulder seams in the yoke facings.
The first thing to do is to sew the neck seam, understitch it on the facing side and then clip and press (this is the order that works best for me - and it give the smoothest results).
This next part involves a little maths. If you use seam allowances other than 1.5cm (5/8") then you will need to work this out for yourself. My little brain took ages to figure it out for 1.5cm!
Trim 0.5cm off the armhole edge of your facings, this will ensure that the facing rolls nicely to the inside of the top.
Next, open the top out so it looks like the picture below. The shoulder seams are running horizontally. Forgive my photo, I don't have photo editing software so I had to do this in crappy old paint.
Get hold of the armhole ends of the shoulder seams, facing in your left hand and yoke in your right. Bring your left hand around behind the top and match the seam ends in your right hand then pin them together. Don't think about this too much - it will hurt your brain. The top will resemble a fortune cookie at this point.
Work your way out from the seam until the entire facing edge is pinned to the yoke edge. You will need to wiggle and adjust as you go but it can be done. If the entire armhole is faced, you'll need to do this in two goes.
Sew in a 1cm seam (not 1.5cm because of what you trimmed off earlier). Your seam will not match up to the finished edge of the lower armhole but when you turn everything through to the right side you will find the yoke and facing roll nicely to the inside and look really smooth and neat. You can understitch this seam too. It is tricky but worth the patience. This is the finished armhole from the inside.
The next stage is to sew down the facings to the yoke seams on the inside. You can turn under the edge and stitch in the ditch, which is what I planned to do, but in the end I decided to hand sew this part. It really didn't take long and gave a really nice finish.
Here is the finished top. I made a narrow rolled hem on the bottom. Despite my best efforts it's a bit wonky but I intend to only wear it tucked in.
Here it is tucked in, as I'll wear it to work, bear in mind that I am a little larger than my dummy.
I'm pretty pleased with the way this top turned out. The silk satin is beautiful and not too shiny for daytime. There are a couple of creases that I can't shift so I am going to have to take it to the drycleaner to press it for me but after that it should be fairly easy-care since I pre-washed the fabric.