For my purple dress I followed my toile and added a halterneck strap to do that I drew out the shape that I wanted and used that as a pattern, I added 1cm SA and cut it out, i then sewed the pieces together and bagged it out and pressed. I also sewed the hem using a special foot that tucks the edge under for you.
Here I have only pinned the strap to the dress incase it doesnt fit when I go to fit my model.
Thanks for reading!!