Well, you really have to mess with all the features of Photoshop yourself to learn how to do it, but I will give you a few ideas to get you started.
First, you have to open up all the different image files in photoshop (the main skin, the gloss file, and the norm file). In my case, I even opened up the old Hell Knight skin file, for the original color of the hell knight. Next, copy one of the skin files, such as the gloss file, and paste it on top of the main skin. I can't remember which order I pasted them for this particular skin, but you would have to mess with the order yourself. When you paste, you will see the gloss file (or whatever you chose) completely cover the old skin. Now, select the skin you pasted in the layer window, and you will notice it is set to "normal". Change this mode to something else, such as "saturation", or "overlay", and watch how the original skin changes as it blends with the one you pasted over it. You also have to mess with the opacity and fill levels, so the skin doesn't look too extreme. It takes a lot of messing around to get the skin looking like it should, while matching the color of the original monster. You can try any combination of pasting the different skin files over each other and see what you come up with. If you wanted to, you could even make a gloss file yourself and paste it over an original monster skin and mess with the layer modes.
When I did the Hell Knight skin, I edited more than just the layer and blend modes. There were parts of the original skin I didn't like (such as the crotch of the Hell Knight), so I did actually change the skin itself using airbrushes, the pencil tool, shading, etc.
Hope this helps