The photo editor I used is ADOBE Photoshop CS4, but other graphic editors can be used with similar tools and steps.
Step 1: Getting started
The FIRST STEP is to find two images/photos.
Photo 1: Is the image that you will wish to manipulate
Photo 2: Will be a creepy face image that you will overlay on top of the first one to create your Halloween portrait.
For Photo 2, I usually type into a search engine "Halloween masks." More then enough will come up. Select one that fits the theme you wish to create, and is the same size or larger than the face you will be altering. To make editing easier, select one that has a background that you can easily separate from the main image. Select one where the angle orientation matches up with the angle of the original face. EXAMPLE: If someone is looking straight ahead, to the left, or to the right... then make sure your "creepy overlay" is oriented the same way.
For demonstration I chose a Victorian era butcher, who is outfitted with his saw, cleaver and large knife. I also selected a ghoulish old man's mask for the creepy overlay. Vintage photos like this make excellent choices for your Halloween Horror Portrait.