After I found myself a suitable source pic
(1), I first used Hue/Saturation, to decrease the overall saturation and make the colours greener, and did this especially near the lips
(2). For the head, I looked for a picture of a skull which was taken from roughly the same angle
(3), and used a layer mask to make it seem as if her eye had been taken out.
Adding details
In the neck area (1), I used source pictures of a leg with spider veins (2), a mummy and a war victim (3). Source pictures which are usable for such a contest aren’t always pleasant to look at… I experimented a lot with different blend modes, till it looked right. I finally decided the features of her body wheren’t clear enough, so I did some masking adjustments to bring them back (4).
Dirty Laundry
The clothes (1) were soiled with a soft brush in red and greenish black colours, while using the overlay layer mode (2).
Finally it was time for some more detailed work. I took the image from (1) to (2) by, among other things:
- drawing little zits and such- using hue/saturation very selectively to tone down the saturation and make some skin extremely green- desaturating the hair completely- using overlay, soft light and color burn layers and soft red and black brushes to add more shadow and bloody parts
Hope this helps you a bit. Be creative and have fun! 
Many thanks for the great posting. I am glad I have taken the time to see this.