Carefully cut out the design(s) you want to place on the mirror. They can either stand alone or overlap like a collage.
Apply a thin layer of Mod Podge to the backside of the paper cutouts and carefully place on the front of the mirror. Leave overlap if you like.
Repeat with additional pieces as desired. If any Mod Podge smears on any other areas of the mirror, quickly wipe it away with a damp towel.
Once your design is final, apply another thin layer of Mod Podge on top of the paper, painting it so that there is only a very thin border of Mod Podge around the paper. Allow to dry completely.
If desired, apply hot glue to the back of the mirror and stick to a piece of balsa wood that is slightly larger than the mirror for an added accent.
Video
Notes
Tip: use a razor blade or craft knife to remove excess Mod Podge from the mirror surface once it's dried.