Let's set up your file first. Open up the image in Photoshop. If your Background Layer is locked, double-click to unlock it.
Step 1: Duplicate layer
Right-click on the layer and select "Duplicate Layer..."
Step 2: Keep yourself organized
It's good Photoshop practice to keep your layers organized. Rename them to reflect what they are.
Step 3: Change layer style
Select the Softened layer and change its style to "Overlay".
Step 4: Apply a filter
From the menu, click on Filter > Other > High Pass.
Step 5: Set High Pass
This step will vary a little bit depending on your file size and nature. I set my radius to 6.0 pixels. If you're using a higher-resolution image, set your radius to 10.0 pixels. When you get a decent pass, click OK.
Step 6: Invert
From the menu, click on Image > Adjustments > Invert.
Step 7: Add layer mask
With your Softened layer selected, click on the "Add Layer Mask" icon. It's located below your layers palette, third icon from the left. The icon looks like a circle in a rectangle.
Step 8: Change to B&W
From the menu, click on Image > Adjustments > Black & White.
Step 9: Erase the excess
This step is a little tedious. In your Softened layer, there should be two boxes. Click on the white one. Using the Eraser tool, erase anything that is not skin. Use the Zoom tool to get into the little spots.
Step 10: Almost there
Your image should start to look a little something like this.
Step 11: Final step: changing opacity
One more step to go! Change the Opacity of the Softened layer to 75%. Ta-da, you're done!