It was very easy to do. I figured out how to do it one day at work--I was playing with the fabric belt on my dress, and all of a sudden, triangle pleats! I was quite relieved, as making about 9,000 little triangles was far from appealing!
So, here it is, how to make triangle pleated trim!
First, iron down the edges of the fabric. Just a little--it's too thick if you fold it to the middle like bias tape.
Iron so it meets in the middle. This can of course be skipped with ribbon.