Page 151 - Photoshop for Lightroom Users – Scott Kelby 2nd Edition

Step Three:
Now, in the Layers panel, click-and-drag the no clouds image to the top of the layer stack (as seen below right. The city image is now on top, and the
clouds are on the layer below). Double-click directly on the top layer’s thumbnail to bring up the Blending Options in the Layer Style dialog (seen here
bottom left). At the bottom center are the Blend If sliders (the gradients with triangle-shaped sliders beneath them). By default, the Blend If pop-up menu is
set to Gray, but since we’re replacing a sky, we’ll need to switch to the Blue channel. Now (and this is very important), we’re going to drag the top right
slider to the left, but before we do, we’re going to press-and-hold the Option (PC: Alt) key because if don’t, when we drag that slider, we won’t get a
smooth blending of the new sky. Instead, we’ll get harsh, jaggy results that look pretty awful. So, press-and-hold the Option key, drag the top-right slider to
the left, and you’ll notice that it splits the slider knob in half, which creates a smooth blend. The farther you drag, the more the blue sky from the bottom
layer shows through (as seen here). When you’re done, click OK.