Seams with crossing seamlines
Sewing Rule: When sewing seams with crossing seamlines, all crossing seams should be stitched and finished before sewing the parent seam. The best example of a seam that features multiple vertical crossing seams is a conventional waistline seam. A waistline seam houses the garment's side seams, princess seams and/or center front/center back seam. All these vertical intersecting seams have to be stitched first before finally completing the waist seam.
In addition to stitching them crossing seams, they should also be clean finished and ironed flat as indicated in your project. It is also important to note that two separate seams that are stitches together within another seam, should be ironed and clean finished the same way unless otherwise specified by your project. This will ensure that the two seams are easy to accurately align as well as result in a smoother crossing seam area.