Ruffles! I love ruffles! So I have the ruffler and the gathering foot. But this is about the gathering foot. As usual, here's the Bernina video:
This one actually gives you pretty close to everything you need to know. But you have to watch for it, it doesn't just tell you.
So, if you want to gather the layer of fabric and do nothing else, put it UNDER the presser foot just like normal. It will for all intents and purposes randomly pull the fabric closer together as you sew. The longer the stitch, the deeper the ruffle.
If you want to attach it in the one operation that they discuss, take note of the little slot it has. That's where the non-ruffled piece goes. Right sides together.
Watch out for the gathered piece to move a bit. But that's really the only tricky thing. It's pretty easy to use.