I always have struggled with these, and as you can see from the puckering, I still need a lot of practice.
Before begining, set your machine to "needle down", if it has this feature. My old machine did not. My new one does. It looks like this
Place convex (no hint on this, just remember its not concave) on the first piece with wrong side up, meeting edges at top. I’ve conveniently marked the wrong side with a magic marker. Sorry not to have thought of that little trick before. I’m new to this.
Start stitching, as in Plain Seam and continue to sew to where the edges of the fabric begin to diverge. Make sure needle is in fabric, and raise presser foot.
Press entire curvy seam to one side
Hopefully I’ll get better at this
The Book says: "A Curves Seam , in most cases, creates the style line of the garment, such as on a princess seam, bodice yokes or shirt yokes."