1 Convert to a Smart Filter
Duplicate your Background Layer (Layer>Duplicate Layer) and go to Filter>Convert to Smart Filter to turn that layer into a smart object. The benefit of this is that when you apply the motion blur later, you can double-click on the 'smart' layer to re-edit the effect, rather than having to delete the layer to start over again.
2 Apply Blur
Before you apply any blur make sure the lines in your image are straight, whether they’re horizontal or vertical. For this you can use the Crop Tool located in the toolbar or the Straighten Tool in the Options bar. Once you add blur, wonky lines will only be exaggerated. With the new layer selected, go to Filter>Blur>Motion Blur.
3 Refine the blur
When you open the Motion Blur preferences, the Angle enables you to adjust the direction of the blur. By default, it will be set to 0°, ideal if you want horizontal movement; if you want vertical blur, change this to 90°. The Distance slider controls the amount of blur, which would otherwise be how fast you move the camera.
4 Adjust the Distance Play
around with the Distance slider until you find a suitable level of blur. The higher the number of pixels that are blurred, the stronger the streak or colour will be and the less recognisable the original scene will look. Around 200px, depending on the size of your image, is usually a good place to start.
5 Apply your gradient
To selectively apply the effect to only a portion of the image, click the Add Layer Mask icon at the bottom of the Layers palette. Then use the Gradient Tool in the toolbar, set to Foreground to Transparent, to click and drag from top to bottom or bottom to top of the image (make sure you’re working on the new Layer Mask).
6 Play with the gradient
Whatever areas are rendered black on the Layer Mask
will reveal the original scene, hiding any blur; areas of white where the gradient hasn’t been applied will be blurred. Experiment with how you apply the gradient and blur – it can be a soft transition or hard switch, or you may prefer just having the blur in the middle.