Step 1: Supplies
Optional supplies include: painter's tape (if you can't remove your mirror from its frame), blending gel or craft paint extender, fine glitter
Step 2: Haunting
If your design will extend to the edge of the frame, remove your mirror from its frame, if possible. If this is not possible or if it is really difficult, mask off the edges of your mirror to prevent painting glue onto them.
Using the point of your glue bottle or a brush loaded with glue, paint an outline of your ghost, including eye and mouth holes onto the plain side of your mirror glass. Work quickly--no need to be fussy or exact. Then use your brush, loaded with glue, to fill the rest of the design. Work fast and smooth out the glue so it's relatively even (unless you don't want it to be!)
Let the paint dry and replace the mirror in your frame. If you've taped-off the frame, you might want to use a sharp craft knife the trim any glue overlapping onto the tape before removing it.