This is a simple, fast way to add just that extra element of spookiness to your Halloween decor. You've probably got everything you need already. So let's get started!!

Step 1: Supplies

You need: a mirror, some white glue (school glue works just fine) and a paint brush.
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

Decide what ghostly image you'd like to paint. Keep it simple. A head, a body, maybe and arm or two will work just fine. Play with the design a little. This process is so simple that you can repeat it over and over without wasting much in the way of materials or time. It takes just a few minutes to create your ghost (but perhaps an hour to dry).

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.

SIMPLE!

Step 3: Options

There are plenty of options here. For increased working time for spreading the glue, you can add blending gel or paint extender. You can add a little or a lot to the glue -- up to half gel, half glue. I used a craft acrylic blending gel. Because it thins out the glue, it results in a less opaque ghostly image.
Glitter! While your glue is still wet, you can sprinkle very fine glitter on it, in whatever color you'd like.
Add just a touch of color to your glue with a few drops of acrylic craft paint. Make a filthy gray ghost, a luminescent ghost with glow-in-the-dark paint, or make a funky purple specter. It's up to you!
Don't use a mirror. These ghosts can be added to any piece of glass-framed art. Just paint your ghost onto the outside of the glass. Is that a ghost trapped in that picture?! Spooky!!