You may get .psd* file of "Unusual Christmas Card" Photoshop tutorial by making a payment. Put the tutorial title in "Add special instructions to seller" line of PayPal and it will be sent to you shortly.
- Draw light soft spot between background and card layers.
- Apply Soft Light Blending mode to it to create smooth lightning effect.
- You should get something like on the screen shot below.
- Create selected shape above background layer using Pan Tool.
- Fill it wit black/transparent gradient, play with Opacity, apply Gaussian Blur Filter (around 3%)
- Insert cloud image.
- Separate it from background. You may found detailed tutorial, how to remove background here: http://alfoart.com/green_planet_3.html
- Erase superfluous areas using soft Eraser Tool.