Oh you don't need a selection for this. Just put the tree layer on top of the sky layer and set the blend mode to Darken.
All that being said, no one has mentioned the HARDEST AND MOST DIFFICULT TO MASTER step in the whole process. That step is, "Using a sky shot that matches the main shot." You will soon discover the disappointment of dropping just any set of blue sky/puffy clouds into any gray sky and have it look like crap, but you don't quite know why. In order to do this well, the shots should match - or at least be believable - on many different criteria, including but not limited to:
Lighting direction. If the clouds are lit from the right and the main shot is let from the left (or not at all), it will look obviously fake.
Color temperature. If the clouds are from a nice golden sunset and the main shot is from a cold December morning, it will look obviously fake.
Perspective. If the clouds were taken with a 28mm wide angle lens, and the main shot is from a from a 200mm telephoto, (or verse visa) it will look obviously fake.
Tons of other things I don't want to bother typing. If the clouds are one thing, and the main shot is another, it will look obviously fake.
Rad