This shows the back side with the folded connector strip sewn to the back. The second section is sewn to the front connector strip, with a 1/4" seam allowance.
The wider back connector strip will cover the butted seams made by the front connector strip.
Fold the back strip over the seams, and topstitch from the back or pin carefully from the front and stitch in the ditch, catching the edge of the folded strip.
Here the finished look. If I had used solid fabric, it would have been nicer, but the striped strip makes it more understandable.
The two choices of covering the seam: folded strip on the front or folded strip on the back. Either one works, but one is more visible on the good side while being easier to stitch. It's up to you which to do.
This shows the doubled connector strip on the back, and the batting is cut to fit in between the seams, and then zigzagged to one edge of the seam allowance. Fold over the back strip and topstitch, either from the back or the front.
The simplest one of all is the Flip and Sew border technique. Join a top strip and a back strip (not folded!) at the same time. Then add batting strip, and zigzag butting it up against the seam allowance. Then quilt the strips.
Please feel free to leave any questions in the comment section.