Now we need to make the ‘B&W Skull’’ layer black and white: hit Cmd/Ctrl + U and desaturate the layer. You
may also want to tweak the skull’s levels to make the black a ‘true’ black – but you can always do this at the end instead.
Add a mask to the ‘B&W Skull’ layer by clicking the Layer Mask button (it’s the icon featuring the circle inside a square at the bottom of the Layers palette).
Select the white box that appears on the ‘B&W Skull’ layer once you’ve created the layer mask by clicking on it, then grab the Brush tool (B) and set it to around 250 pixels. Paint around the skull to mask off the bits we don’t need. This doesn’t have to be exact – it just helps you concentrate on the central bit alone. Set the layer’s opacity to 50%.