Follow the diagram to understand the same:
Make a duplicate copy of the layer (ctl/Cmd+J) and rename it as Gaussian blur.
Go to the filters menu and select Gaussian Blur as shown in the image
Keep the % upto 23 roughly.
Reduce the opacity of the layer to about 37% roughly. (you can reduce more to see how you like the soft focus effect)
Add a layer mask to the Gausssian Blur layer from below. Now select the Brush Tool from the tools panel and brush on the areas that you want sharp from the rest of the soft focus areas (like eyes, lips, nose, etc)
Please note : that If there is a white Mask (default) you must paint with Black color. And vice versa.