In this instance I have a white tatted flower and my ribbon is white so I can begin by using ordinary white thread. Open the barrette and remove the centre spring by bending it a little and pulling it out first on one side, then on the other. Put it aside while you work on the bar.
Measure off a length of ribbon about 2.5 times the length of the bar. In this case it's about 8 inches. You need to have more than enough to go completely around the bar. Notice that this ribbon happens to have a pattern of white dots on it. Don't worry about tone on tone pattern as it won't really be noticeable when you're done.
Insert the ribbon under the clasp and pull it about half way down the inside of the barrette as shown here. Wrap the other end around the barrette and insert it under the hinge at the opposite end of the barrette like this. Fold the end about a half inch so that the cut end is folded under
and begin to stitch the end down making sure that it is wrapped snugly around the bar. Shiny metal and shiny white ribbon don't photograph well,
but you can see the fold and the beginnings of the first stitch.
Work across the centre seam to the edge of the ribbon and whip stitch along the edge just catching the outer edge of the ribbon on both the top and the bottom as shown below.
When you reach the end slide the needle under the ribbon and out the other side and continue whip stitching down the second side of the barrette. Here is the completed bar with the ribbon sewn down both sides.
If sewing isn't something you want to do you can attach the ribbon by using a glue gun, just make sure that you don't get any glue on the top side of the barrette where it might show. Glue down the first side, then fold under the second end and make sure the ribbon is tight before you apply the glue.
This pattern has green leaves as well as a white flower and white thread is going to very noticeable on the dark green The invisible nylon thread works with any colour of thread and it's invisible when its on.
Tie a knot in the end of the thread and slip the needle under the ribbon on the top side of the bar bring the needle out in the middle on the ribbon close to one end go through the tatting and the middle of one end and then insert the needle under the ribbon again and take a stitch. Work along the bar taking a stitch in the ribbon and a stitch in the lace.
Add in the second leaf and continue tacking it down all along the length of the bar, as shown here. You may not be using leaves and flowers, but the process is the same.
The last piece to be added is the white flower. Stitch through the middle of the flower and and into the ribbon tacking it in several places. Tie a knot in the thread feed the needle through the ribbon out through the side and cut.
Sometimes, depending on what it is I am attaching, I just leave long ends on the tatting and wrap them around the ribbon covered barrette and literally tie the lace onto the barrette criss crossing the thread around the bar and back through the tatting. This method, although not quite as secure, has the bonus of not having to hide the thread ends.
Again, if sewing's not your thing, you can use a hot glue gun, but you take the risk of damaging your lace. If that isn't a concern then go ahead and glue it.