A couple of weeks ago I got an question by e-mail (sorry Lois I am notoriously slow in answering my email) asking how I make my neckhole. I couldn't explain in words so here is my attempt with pictures. I hope this is helpful.
keyhole neckline in tunic.
Measure by laying the liningfabric (linnen,silk or wool) into the garment and draw the opening with chalk.
Add a couple of centimeters to make outside of the lining and cut.
Pin to the outside of the costume
Sew and zigzag, turn the lining in, covering the seam and edge.
Now you are working on the inside of the garment.
Fold raw edges in and sew by hand, be careful to pick-up a minimal amount of threads of the garment fabric to make visibility on the outside minimal.