Stitch sets of three vertical running stitches on top of each other. Note that you start the second triplet directly above the last stitch (indicated by the second red #1.)
Introduce the second thread color at brown point #1.
Thread the second thread color through the first line of stitches as shown above.
Curve the second thread all the way up the first line of stitches.
Introduce another piece of the second thread color to the left side of the stitching and thread it all the way up the line of stitching , mirroring the curves of the right side.