Edited to add: I've made a pdf file for these instructions. You can find it here.
Before adding the shoulderstrap, you should measure your closing at center back. The shoulderstrap should be within the closing. In the picture I've marked it with the line. So I needed to trim it a little bit. When your hook and eye closure is wider than the back, you pin your shoulderstrap a bit out of the fabric.
Cut your shoulderstraps in half. Don't cut a separate piece for the back, leave it as one part.
Pin the shoulderband to the back and stitch it with your widest zigzag.
Trim the fabric on the inside.
Take the ring and pin it. Sew with a narrow zigzag and very close stitches. I made two rows of zigzag stitches, to make it more secure.
Cut off the shoulderstrap, you now have the strap that you will attach to the cup.
In the same way, with small zigzag stitches, attach the shoulderstrap to the slider.
Slide the shoulderstrap through the ring, from the back to the front, and then through the slider.
Pin it to the cup
Stitch, again using small zigzag stitches.
Close the wire casing on both sides with narrow zigzag stitches too, and cut off the extra length that is sticking out still.
The bra is finished, add a ribbon decoration if you like.
I really hope that this tutorial is helpful to some of you. Any comments are welcome. I tried hard to have all the English bra vocabulary correct by searching these on the internet, but please let me know in the comments or by e-mail if I have made a (stupid) mistake or if I could make it clearer by using another word.