Rendering Sky as Alpha

I’m aiming to map some of my renders onto planes to create Disney-style mutliplane shots, and I was wondering where I could learn how to automatically replace the sky with an alpha channel, so leaving me with less work to do in the GIMP.

If it helps, I’ll say that my knowledge of Blender is limited to the 5 current episodes of the “Blender Basics” tutorials from Blender underground. So by asking this question, I might be revealing even more ignorance than I am aware! Anyway, thanks!

Well I know if you change the render format to PNG, it automatically replaces the background with an alpha channel when you save it.

see the part about sky, premul and key.

sky is always alpha 0

Thank you for your help!