To insure that there is no curling or fading, IMHO
I would pre wash the curtains, if washable, and the ribbon.

The site you posted said it is Grosgrain ribbon, Satin would probably not hold up as well.

Probably would use something like Steam A Seam light, use a press cloth, this will hold ribbon in place and straight, then stitch close to the edge of ribbon.

Might also try invisible thread for the top stitching if you cannot exactly match the color you are using.

They also have a tape used in quilting that is sticky on both sides and disappears after laundering. Sorry do not remember the name.
Hopefully another member has input.