First of all I cut out the lips using polygonal lasso tool.
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Upper and lower lips were cut out and a layer for each one was created via copy. I created another copy just in case something went wrong.
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I placed the skull in a new layer at the bottom, just above the background, rasterized it and worked on it as to adjust the teeth to the mouth.
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Using the warp modes I worked on the lips to open and distort the mouth and show the teeth [dont't bother with the colour burn you see in the image below. It comes later. I forgot to make a copy of the distorted lips :-)]
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I applied shadow, some colour and hue/saturation adjustment to the skull.
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Now the zipper. First I created the pieces of the zipper [you can go to http://www.webdesign.org/web/photoshop/drawing-techniques/zipper-tutorial.16469.html to learn how to draw a zipper. Mine is far from being so nice.]
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I created a folder named 'Zipper' and two other folders within, one for the zipper on he upper lip, and oher for the zipper on the lower lip. I duplicated the first piece of the zipper and placed it on the folder 'ZIP 1' and made as many copies as needed to place on the upper lip one by one following the curve. The same process was used for the lower lip. In each zip folder I created a layer for the shadow.
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I placed the other pieces of the zipper inside the zipper folder.
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Below the zipper folder I created two adjusment layers, one for photo filtre and the other for hue/saturation. Worked on shadows and colour burn.
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I created a new layer and merged all visible layers [shift+ctrl+alt-e] and duplicated this layer.