First attach the grosgrain ribbon by top stitching it onto the seam allowance (I use a 6mm allowance). I use a blind hem presser foot to make the seam more precise.
The waist after the top stitching is done. Look how discreet the seam is.
On the the flip side. Not as slick, but it will soon disappear. I created a self fabric hook for the button.
Fold the grosgrain ribbon over and press into place.
To secure the ribbon I sewed a line of stitches in the ditch of each seam (including the darts).
What it looks on the other side. This will keep the ribbon in place.
The finished shorts. I used the them plenty in Portugal and the ribbon facing really kept the waist from growing. I normally have problems with this, even though I use interfacing and stay stitching.