1. duplicate a layer with all the other PP work done
2. you can either apply gaussian blur or surface blur whichever looks good to you. don't worry if the preview looks like all the details are being lost, that would be fixed later at step 5.
3. you would want features such as the eyes and mouth to retain sharpness, so first add a layer mask.
4. next, use a black soft edged brush tool and apply to the areas you would want to remain sharp (eyes, mouth). what you're doing is simply revealing the layer beneath that does not have the softening effect caused by the gaussian/surface blur.
5. finally adjust the opacity of the layer so as to determine how much of the softening effect you want on the skin.