Trace the shape of your box onto your fabric and cut it out. It's better for it to be too big than too small.
After you do this, use mod podge to paint it into the cardboard heart box. Use the box's top.
Allow the mod podge to dry. After this, use the hot glue to attach the mirror wherever you want it to go in the box. I attached mine dead center.
Add whatever trim or other embellishments you desire.
Viola! You're done.