I'm still learning the technique myself but basically you open up a layer the same size as your picture and add some type of gradient to it...you can play with the colors, brightness, contrast, saturation, tint, hue, etc. until you get something you like and think will fit the pic...then you open up the original still as a layer on top of your gradient layer...then you select regions by color (this is how I've been doing them I suppose you could use other things like the magic wand for example) selecting for the white burnt out sky area or background or whatever area you want your gradient to be in...then you cut that away...merge the layers...flatten the image...then save...you have a processed picture. I'm really sort of just beginning with gradients your pic above is one of the very first I have ever attempted...