Use two standard sewing threads in a size 14 needle (or larger) to match the topstitching on the jeans.
Topstitch the fabric near the fold of the seam. Use a ‘hump jumper’ or a folded paper spacer to sew the intersection of the seams. (See the gray object to the rear of the presser foot in the photo.) Otherwise, you will not be able to advance the presser foot.