Tutorial replacing skies behind trees/with trees on horizon?

I have replaced hundreds upon hundreds of skies and my first few thousand look pretty amateurish to say the least.

I finally figured out that a vital part of the success recipe is the source sky one uses and how that sky was captured; also, scaling the detail in the replacement sky to the "zoom level" of the target picture goes a long way toward realism and whether it is obvious that a sky has been replaced.

A beautiful dramatic sky, or an interesting sky with a lot of dramatic detail, only ever looks real in the scene where it occurred; these types of skies are not good choices for replacements; likewise a sky that has a big cloud that looks like Abraham Lincoln.

Ocean skies seldom look real in a landscape unless it is an arid desert scene.

A convincing replacement sky must have the horizon included at the "bottom" of the sky; having a faint faded horizon line on the replacement makes blending it into the new picture a lot easier and with a much more realistic result.

Whenever you are in one of those rare places where the horizon is straight, clean, and unbroken, such as a long level tree-line on a distant ridge, and the sky looks appealing, catch the horizon line in the bottom 15% or so of the frame; it helps to have a graduated ND or CPL filter.

Returning to this same place several hours later, or on different days, will yield completely different sky scenes so all of your pictures won't have the same clouds in them; that being said, any given set of pictures taken in the same general direction will have the same sky and this should be taken into consideration to preserve believability.

I hope this is helpful.