Add the Birds

1. Select Layer -> New Layer from the menu and enter the name Birds and press OK
2. Select the Brush tool by pressing the B key
3. Open the brush editor and click on the settings drop down (looks like a gear at the right of the screen)
   
4. Select Load brushes and browse to the Flock_of_Birds_by_Ubershinysheep.abr you downloaded from the resources above.
5. You should see a birds added into the brush panel
6. Choose a group of birds that you like and adjust the size of the brush as you like
7. Set the color of the brush to black and the Opacity to 100%
8. With a single click paint the birds onto the Birds layer
   
9. Double click on the Birds layer to open the Layer Styles window
10. Set the opacity to around 20% so the appear to be a long way away, seen through the clouds and press OK
11. From the Filter menu select Blur -> Gaussian Blur
12. Enter a Radius of 2px and press OK. This will give the birds a slight blur as they would be seen from a distance

Add a slight vignette

1. Select Layer -> New Layer from the menu and enter the name Vignette and press OK
2. Select the Brush tool by pressing B and open the Brush toolbox
3. Choose a normal round brush with a hardness of 0% and a size of 1500px
4. Choose Black for the brush color
5. Set the brush opacity to 20%
6. Paint around the edges of the image in black to darken the edges
7. In the layers panel double click on the Vignette layer to open the Layer Styles window
8. Set the Blend Mode drop down to Soft Light

Finishing it off

At this point you can spend a little time adding some contrast with an adjustment layer or moving some of the elements around to help with the composition.