MTS

You just fold over the rows. They're perfectly aligned - assuming you placed the rectangles correctly - that if you fold and sew on the line provided, everything will match up perfectly. Hey, an engineer designed it. ;-) The interfacing stays in and together as one piece.

A little pin here and there won't harm the process ;-).

You can do a very large basting stitch (an old Carol Doak trick) if you wanted to see if the diamonds match up correctly before you commit. I will say that it is a royal PITA to do any unpicking of small stitches on that interfacing so be sure before you sew.

Once all the rows in a section were all sewn together, I split the seams and pressed them open, but that's just a personal preference. It's not necessary.