I have not used the Hearts a Flutter ones but have used other SU Framelits in my CB. The sandwich that I use is...bottom to top...
Die (cut side down)
Some people switch them around so that the die is on the B plate (cut side UP) and then paper and then C plate but since my B plates are all already messed up from cutting, this one keeps my C plate prettier
Plus if you are framing around an image that is stamped, for me, it's easier to line it up with the Framelit on top rather than on the bottom