For this manipulation I will start with 3 main photos....the main character, skull overlay and leaves as the background.

The first thing I want to do is to add a layer mask to the main subject. I do this by making sure that layer is selected and by clicking on the add layer mask icon at the bottom of the layer pallet.

Initially the mask is white and nothing is masked. I clicked on the mask of the main character and using the paint brush and black being my foreground color, I mask away around the subject to let the background show through.

Next I add a layer mask to the skull layer that is already in Overlay blend mode. I click on the mask and instead of painting with black I fill it with black by hitting (alt + del). Now I switch the foreground color to white and paint in the parts of the skull I want to re vial.

Note : I can now go back and repaint any of the mask to show or hide what I want as many times that I want till I get the effect I'm after.

This the result I have now from step 2.

In this step I'm going to add adjustment layers so I can adjust the overall contrast and Saturation of the manipulation. With the top layer selected click the adjustment layer icon at the bottom of the layers pallet and select the adjustment you want to make.

Any adjustments you choose automatically comes with its own mask. If you notice that in the levels layer mask I painted small gray areas to tone down the adjustment in those areas. I did this by turning down the opacity and flow in the paint brush options.

This the result I have now from step 4.

I didn't like the background the way it is so selected it and add 3 different layer adjustments. The Channel Mixer to take the color out, another levels to adjust the contrast and solid color adjustment and changed the blending mode to Color to add a green hue.

This the result I have now from step 5.

1. I wanted to make the pupil of the eyes lighter so I added yet another levels adjustment layer and filled the mask with black. Selected the mask and painted the area of the pupils with white. Then simply adjusted the levels to make the eyes lighter.

2. I added a black vignette to the background by adding a new layer right above it and filling it with black. Then I added a layer mask selected it, filled it with black and paint the areas I wanted the vignette to show with white.

This the result I have now from step 7.

Here I added a object that I'm calling Jewelry. I don't know that it really goes well with this manipulation but for demonstration purposes it will have to do.

My Jewelry needs some tweaking so that it looks like its part of the manipulation.Using That is where clipping masks come in.

With the jewelry layer selected I Alt+clicked the layer adjustment icon at the bottom of the layers pallet. A dialog box will appear and check the box "Use Previous layer to create clipping mask" This will create an adjustment layer that is also a clipping mask.

Clipping masks will only effect the layer that its assigned to and not the entire image.

I added a curves clipping mask to lower the contrast of the jewelry to match that of the underlying photos.

The jewelry need some shading so instead of using the burn tool to make the outer part darker, I created a new layer above the jewelry. I selected the new layer and pressed (Control+G) to convert it to a clipping mask.

I then painted black in the area I wanted to darken and turned down the opacity. Since this is a clipping mask the black will only show on the jewelry and not the background.

Here's the final image. The point I wanted to make with this tutorial is that by using adjustment layers, layer masks and clipping masks you have the ability to go back and fine tune your work.

If you to use tools like the eraser and burn or dodge you would be committed and have no recourse other than to start over.

I hope that using these techniques will help improve your future manipulations because the difference between a good manipulation and a great one is details.