Or, you could do like I did in this card and just buy ribbon that's already that way...
Friendship Card by kikikaren at Splitcoaststampers
I've also pleated it using tape, but you need to use a tape that's strong enough to hold it. I've used Snail to do this a couple of years ago and it fell apart within minutes. Scor-Tape is probably the best choice for tape on this. You can also use staples. Tim Holtz's Tiny Attacher would work well as the staples are small enough to hide and the stapler is stronger than mini staplers so the staples will actually work and not jam into the stapler. But, of course, you'll either need to do that on a top layer and then put a layer behind to hide the staples (assuming you're stapling the ribbon to the CS) or just suck it up and deal with having staple backs on the insides of your card. Sewing is also another option, especially if you have a machine that will gather for you, making it easier. Or, you could even hand stitch it with large stitches and pull it by hand.
Bunched up ribbons?
