I hope this works!
This is what I started with a square shape and a curved edge
The red stitching is the stay stitching. It is inside the seam allowance ( which is the purple dots)
here is where you snip into the seam allowance. I have notched this out but normally i would just make a snip into it almost to the stitching.
Then pin it together stretching it gently around the square side. You will still have a corner poking out a little
then stitch together. When you get to the curve pull the little gathers out of the way towards the back. Its easier to do this with the needle down.
This has been stitched. You now have 2 stitching lines
trim your seam allowance....
turn right side out and press! It doesnt matter if you are making bags or clothes the process is the same. It does take some practice.......a good chance to use up scraps!
i made this from scraps. its basically a Jordy but the outside is pieced together.