1: Flatten the image 2: Resize image to 1.666 times the final size you want. If your final output size is 600px then you should resize at this point to 1000px 3: Apply the Sharpen filter (not Unsharp Mask) 4: Duplicate the layer 5: Apply Sharpen filter (to the top layer, it will look oversharpened) 5: Resize to final output size. So if you have just resized your iamge to 1000px resize it to 600px now. 6: Use a layer mask on the top (sharpest) layer to adjust opacity and mask areas which look oversharpened. 7: You may wish to slightly correct the saturation which will have changed during the process.
BTW I did another way and got even better results as for me. Maybe this will be helpful for you: 1. Duplicate the basic layer with image. 2. Apply Filter -> Other -> High Pass with 0.5 radius. 3. Change the layer blend mode to Overlay. 4. Duplicate Overlay layer. 5. Add mask to second Overlay layer and adjust opacity and mask areas if oversharp.