Holding down CTRL in the Layers palette puts a new layer below. Then with the same layer selected use the filter menu and select blur then go out to Gaussian Blur. In the resulting window choose 16 pixels then do the same again another 3 times reducing the value by half each time. Still with the same layer selected use the keyboard shortcut "Ctrl+Shift+I", clear the layer by hitting the Del key, and finally Deselect the layer.