I've used this little trick several times, and am in the process of recommending it yet again to a new client. If you're looking to cut costs on window coverings but still have a custom look, this is such a great way to personalize off-the-shelf curtains or shades in an budget-friendly way.
These blue curtains in our dining room are from Ikea several years ago. My favorite sources for trim are two local fabric outlets where you can find some amazing deals -- I often see high-quality, fun/beautiful designs at $1.99 or $2.99 at 50% off. Depending on the curtains/windows you're covering, you may need quite a few yards, but I've always been able to find enough yardage for my projects. And even with several yards, it's a much better bargain than fancier or custom window coverings.
This border (below) on our bedroom windows was such an amazing find. The pattern is slightly imperfect - definitely done by hand - which makes it perfect to me! I love it!!
(Sorry about the poor photo quality here -- hard angle to capture but I wanted to give you a sense of the room/windows.)
And in this room for teenage sisters, we used a thin velvet in a fuchsia color. Just enough to give the curtains some definition against the white walls. In this small room, the white walls and curtains really help make it feel more spacious, but the trim brings some extra style and oomph.
On all of these projects, I used fabric glue - so simple!
If you're interested in working together to make your home a place you love, please contact me for more info at firstname.lastname@example.org. I would love to hear from you!