Lets start with this picture
It's an image from Pixabay and we want to remove the birds in the sky. In this case its a very easy job. We can use different tools to do the job, but now we use the Clone Stamp Tool (S) to do it. You just can start on this layer and remove the birds. But it is better to do it on a blank layer, so that in case you messed it up you can easily go back, by removing this layer.
So we add a new blank layer above this layer.
Point the cursor at the image area you want to paint with, hold down [Alt] key, then mouse-click. You have just selected the source point for cloning. Paint over the bird to remove it.
Repeat this steps a few times to get a better result.
Sample and paint for all parts you want to remove from your image.
By clinking the blank layer on and off you can also see the result of the Cloning.