Here is a photo.
It wouldn't be this exactly. I just found a pic online for reference.
I quilt on buttercream all the time. Let your buttercream crust and smooth it out. After it is smooth I let it crust another few minutes. I then place the cake in the freezer for less than a minute. I dust my quilting impression mat with cornstarch and tap the excess out in that minute it is freezing, then use the mat to impress the quilting immediately. Good Luck!
The first one I made I did by hand because I did not know such a thing existed, I was so excited to know there was one. I got mine from a local cake supply shop but this looks like the same thing.