Start with the regular steps of sewing your bodice back, up until the point where you would normally gather the straps. Then take your strap and fold it in half, right sides (main/outer fabric) together, and put it under your presser foot, as shown below.
In the center of your folded strap, sew a few basting stitches (vertically, from the raw edges). *NOTE* For a slightly thinner strap (where it meets the front bodice) you can move the line of stitching slightly left of center (further away from the fold).
Next, flatten the strap back out and distribute the extra fabric evenly on either side of the basted seam, as shown below. Baste along the raw edges to keep it in place.
Finish your Reverie bodice as per usual, and remove your basting stitches. And you’re done!!