Then fold the top down twice to made a channel for the ribbon.
Here is the tool part. This is a simple and inexpensive sewing tool, but such a time and trouble saver when inserting elastic - or ribbon! I believe the technical term for it is "bodkin." Slip the elastic (or ribbon) through the hole and drag it through the casing. Way faster and less hassle than the safety pin method (though this method must still be used for very wide elastic, or for very small circular casings, such as the elastic on the sleeve of a child's shirt or dress. The bodkin is too long.
Unfortunately, there isn't a picture yet of the results. I'm not sure that I'm going to be super-delighted with it on her, but haven't had a chance to really tell yet. I'm thinking that it may be a summer nighty or play dress for the 3yo, or a top for the 6yo.