1- First convert a raw file twice. Once for all the highlight details. Again for all the shadow details. Like this:

2- Now drag the darker version onto the top of the lighter version. Make sure View - Snap is checked. Use the Move Tool  from the Tool Box. Click and drag. As you get close to the edge it will SNAP into place.

3- You now have two Layers. Window - Layers should be open. Activate the lighter version (bottom layer) by clicking on it.  You will see it turn blue indicating it is the active layer.

4- Now hold down Control + Alt + the Tilde key (looks like this ~). This automatically places a luminosity mask in place.  You will see the infamous marching ants showing the area affected. It will look like this:

5- Now activate the top layer (darker layer).  Add Layer Mask by clicking on the third symbol from the left on the bottom of Layer Box. (It looks like a gray square with a white circle in the center.)  Like this: Look for the white curser arrow. 

7- The rest is easy. In the top layer click on the mask. It is the one on the right side. Now add a little Gaussian Blur to the mask to blur the details. You can see the results as you go (Filter-Blur-Gaussian Blur).  Be careful not to add to much as you will get a halo effect. Many photos will not require ANY Gaussian Blur.

8- Now feel free to touch up your image as you see fit. By using the Dodge Tool on the mask itself you can make any portion darker - like the sky part. By using the Burn Tool  on the mask  you will cause the bottom lighter photo to come forward. Add a touch of contrast and saturation and you have this. 

Another use. Notice the windows. Click to enlarge. Look for lower right hand arrows to make even larger.