Here's how I do it:
First I sew the gathered ruffle to the Over Dress with wrong sides together, zigzag it, trim and press towards the Over Dress. Next I trim off the fabric from both sides of the Swiss beading up to the "railroad tracks". I lay the wrong side of the railroad tracks (it's actually a row of entredeux) right over the ruffle seam and stitch with a narrow zigzag. The zigzag is over the straight stitch on the wrong side. And one stitch goes in the entredeux hole and the other swing of the needle goes just into the fabric.
Basic Yoke Baby Ensemble Pattern using the same technique at the yoke seam.