This is a technique I sometimes use: I'd like to do it more often. When I say "frame," I'm not talking about a narrow decorative border, but rather a significant part of the collage. Yet it's impossible to define how wide or what shape the frame should be, because both collages & frames come in all sizes & shapes. They definitely don't have to be rectangular!
Please humor me while I spell out a few different kinds of frames made of paper. (1) One is simple, as in an image (I used photos, here & above) of a frame one might use if the collage were hung on a wall: here's an example. (2) Another is more complex, actually another collage: here's an example where the collaged frame has spooky faces to go with the spooky content of the collage itself. (3) And a frame can even float just inside the outer edge of the collage, as in this one.
Note: a frame that's a collage itself doesn't have to relate, in content or form, to the inner collage. It can be any kind of "extra" collage at all (like #3 above).
I hope this will be fun to do, and maybe a bit of a challenge... =smile=