I made some triangular bound buttonholes for the coat - so here is a small tutorial how to make them. Click on the images to enlarge them.
To start you should take cut a rectangular piece of your fabric and put in onto the place where the buttonhole will be.
You draw the lines as follow - a is the height of your button hole, b is the diameter of the button plus its height.
You stitch along the buttonhole border lines leaving small gaps in order to cut the fabric piece easily there.
You draw the following cutting lines and cut...
Next you fold piece 1 inwards and flatten it with the iron so it stays in place:
Step 6 will be to turn piece 2 over, so you can flatten the edge too:
Fold the piece inwards on the middle line of the buttonhole. You can orient on the middle on the cut pieces - this time you don't fold them inwards too, but you are folding the fabric around them.
Same with piece 3:
You should flatten it again carefully so it is forced into the right shape. Then you stitch the line shown in the drawing in order to fix the pieces. You cut the surplus fabric.
This is what your buttonhole will look like then:
The close the backside you can cut the triangle out without a second piece of fabric onto it or with a very thin fabric - turn it around making a triangular hole and stitch the front and back side of the buttonhole together by hand.
Thanks goes to the vintagesewing and The Sewing Divas where I learnt how to do these.