First, let me say that anything from the fence to the front is real. Anything beyond the fence is phony as heck. The sky, the mountains, the trees, the clouds and anything else you think you might see is all trickery. The reality of the background was just some leafless trees and a nothing sky.
One of the big tricks to doing something like today's image is to put everything on a separate Layer. Because of that need, Adobe Photoshop [PS] (whatever number you have) or Adobe Photoshop Elements [PSE] needs to be employed. As usual, some basic adjustments had been made in Adobe Photoshop Lightroom [LR] (again, whatever number you have) before bringing the basic image over to PS. Because things (the background) were going to be replaced, Layers were needed. If you're new to PS, the biggest recommendation I can make is to use Layers. Lots of Layers. Every time you change Brushes, switch colors, use a Gradient, add components, use a Layer. It just makes life easier down the road. If something on a Layer gets too messed up and you can't figure out how to get yourself out of the jam you've put yourself in, just dump the Layer and replace it with a new one. You'll only be replacing one set of clouds, or one tree, or one "thing" you've added. If you're a more advanced user and have the use of Layers down pat the big tip would be to use Smart Objects. Lots of Smart Objects. Again, it's all about making life easier. Smart Objects are wonderful devices. If you are far enough along with learning PS, make sure Smart Objects are on your list of must use "tools".