https://www.youtube.com/watch?v=ZV9u0Wu8L0M
http://fstoppers.com/sharpening-with-blur-bring-back-insane-detail-with-this-quick-technique
And because no one on the web seems capable of putting together a simple step-by-step tutorial:
- Make 2 copies of the original image (3 layers total)
- Group the top two layers
- Change the group's blending mode to Overlay
- With regards to the group's top layer:
- Blending mode to Vivid Light
- Invert the layer
- Apply a Surface Blur filter (better yet, change it to a smart object with the filter)
Seems like a radius=8 and threshold=10 is a good place to start for the layer.