Take two pictures
1. The first should be empty to create a static background to work from

This is your starter image - you could make yours scarier looking than mine if you want it to be really “ghostly”

2. The second should contain you or the person you’re going to make a “ghost”.

See that creepy looking guy with the menacing gesture - yeah, that's our ghost. Casper's got nothing on this guy.

Use a tripod – it will make the shots line up better. For the second shot set the camera to self-timer mode to give yourself time to get into the frame

Photoshop work
1. Open the two photos in Adobe Photoshop and arrange them so you can see at least part of each photo
2. With the Move Tool (V) click into the photo with your ghost subject
3. While holding shift, click & drag the photo with your ghost subject into the empty photo – this should, if you’ve taken the photos from a tripod or otherwise made the camera sit still, perfectly align the two photos so that everything in the background lines up perfectly.  To check this, change the top layer’s blending mode to difference.  If everything is lined up the only part of the image that won’t be black is the part where your ghost subject is.  Be sure to change the blending mode back to normal before you continue
4. Change the opacity of the top layer (with your ghost subject) to 50%

Top layer's opacity set to 50% - It's already kind of ghostly at this point, but we can do more.

5. At the bottom of the layers palette click the new layer icon to create a new layer.
6. Select the brush tool and be sure that you have a nice round brush and that your brush hardness is set to 0% and change the flow setting to 50%.

Brush Settings - You can see I picked a pretty big brush and set the hardness to 0%. This creates a very feathered soft brush.

7. On the new layer you just created,  paint over the ghost subject with the brush tool in a blue – in this example I picked the following:

Color Settings - you can pick whatever color you want to make your Ghost glow.

The results of brushing over the Ghost subject

H: Now change the blending mode on the blue layer to Screen and watch your ghost appear.

Layer Blending Mode Settings - choose “Scream” – I mean “Screen” from the dropdown list at the top of the layers Palette

Final Results - Not too scary but definitely ghostly.

Play with the blending modes and colors to manipulate the image in any way you like.  Try adding shadows and highlights with lighter and darker colors.

Added some filters (glowing edges, various noise, etc.) The point is have fun - experiment, its the best way to learn how things work.