1. Open the image and duplicate the layer (Cmd or Ctrl J).
2. Choose the Elliptical Marquee Tool and feather it.
The amount of feather depends on your image. But for a large image it can be set around 150px. But see what works best for your picture.
3. Make your selection and inverse it
4. Go to Image > Adjustments > Brightness and contrast
Drag the brightness slider to the left o make it darker.
Easy and fast.