Click on the layer’s thumbnail (the one with the checker board pattern) and while holding down the left mouse button drag and drop this layer on the Create a new layer icon at the bottom of your layers palette and rename this new layer to Black crack by double clicking on its name in the layers palette:

Press Ctrl + i (Command + i on the Mac) to invert the crack; it will now be black.
Important: the crack won’t look black when you look at it in your document window, since the layer’s blending mode is still set to overlay and the opacity is still 75%, since both were copied when we duplicated the White crack layer.

Change the blending mode of the Black crack layer in the layers palette to Color Burn, set the Opacity back to 100% and select a Fill of 85%:

With this layer selected, grab the Move tool in your tool bar and press the cursor right key on your keyboard only once, then press the cursor up key also only once.

At this point you should have something like this:

Notice that by using this method we’re blending the shadow with the actual texture of the wall, which results in fine visible details inside the crack which makes it all look slightly more realistic.

We’re now going to do a little trick to get rid of both identical layer masks, to end up with only one.

First click on the Create a new set iconto create a new layer set (in Photoshop CS2 called a layer group).

Now click on the mask that’s attached to the Black crack layer, hold down the left mouse key, move your mouse cursor until it’s on top of the Add layer mask icon and release your mouse button:

This will copy the layer mask that was attached to the Black crack layer to the layer set, since the layer set was the active layer at that time.

In Photoshop CS2 we can duplicate a mask by holding down the Alt key (Option key on the Mac), click on the mask and drag and drop it onto the target layer:

Next we’re going to remove the masks of the Black crack and White crack layer by dragging (A) each mask to the Delete layer icon at the bottom of the layers palette. When asked “Apply mask to layer before removing?” simply click on the discard button:

After removing the masks, continue by dragging the Black crack layer (B) to the layer set.
Do the same for the White crack layer (C).
Note: don’t drag the White crack layer first and after that the Black crack layer because then you end with the White crack layer as the first layer in our set, which is of course not what we want.

Continue by double clicking on the Set’s name and rename it to Crack:

I’m showing you all this to understand some of the advantages of layer sets/groups. At this point we can easily drag the crack around without having to worry about linking them. We can also easily drag the crack to a different document if we prefer to do so, because it’s now just a matter of dragging the set/group instead of the individual layers. We can now also easily hide the crack by simply clicking on the eye icon in front of the layer. Of course if we have multiple cracks it’s going to be a lot easier to keep the layers palette less cluttered when they’re all in one set/group.

The other advantage, especially for this tutorial, is that we now have a single mask, which allows us to make a single change that will affect both the black and the white crack, which is important if we want to make the beginning or ending part of the crack fade away.

So go ahead, if you want to make some final adjustments to the mask, do that right now.

In the end we should have something like this: