Here is a quick example:



Notice the circled-cross (like a gun target) as I hover the cursor over the image ready to select the cloning point whilst holding down the Alt/Option key.

As soon as I have selected the clone-from point (source point) and let go of the Alt key, the cursor turns to a circle to show it is ready for clone-painting.

Then as I begin to clone-paint, the cursor turns to an arrow to show the point where I am clone-painting. And the point I am cloning from (the source) is marked with a small cross.

BTW skies can be very difficult to clone because skies often have lots of varying subtle hues of blue, grey and off-white which do not always match up well.