The clip art can be scaled for different-size mirrors (first print on plain paper to assess size). This adhesive paper is repositionable: You can peel it away if you need to adjust placement.
Clear sticker project paper, 8 1/2" by 11", $14 for 10 sheets, avery.com
Download clip art; print on adhesive sheets (the images are tiled).
Cut out images. For the skull, cut as close to the images as possible. For the medallion border, cut just inside the thin gray lines.
Before peeling away backing, determine and align the correct position of the border (the design overlaps slightly); with the wax pencil, make small guide marks on the mirror.
Apply border first. Peel away backing, and adhere pieces to mirror. Then center the skull and apply in the same manner.