First, open the photo that you want to use. Go to File > Open, choose your photo, and click Open. Then control or command J to copy a background layer.
Next, Choose Filter > Blur > Motion Blur and adjust the angle to match the direction of your image, creating a sense of movement by adding a blur that flows in one direction. In this case, vertically with the tree line.
Use the distance setting to control the amount of blur that is applied.
Isolate the blur effect by applying a mask layer to keep the detail and using the soft black brush tool to brush over areas that should not be blurred, like in this example, where we want the subject in...