Now a few tips about masking.
Some are not new but I have to use them very often and I think that may be of interest to some GIMP users.
In particular, Dodge and Burn tool for cleaning masks and my new discovery Warp Transform tool for removing fringe has proven to be very helpful for me.
Here is our photo:
I would like to darken the cloudy sky and increase contrast for the landscape a bit. Of course this can be done more effectively with other methods, but we’ll use a mask here.
Since the clouds and landscape hardly differ in brightness we could try to make the separation with monomixer on duplicate layer (Colours – Components - Monomixer):
Increasing blue and green and decreasing red seems to be suitable for separation here.
Let’s raise the contrast now and see what happens:
Separation is better, but we still have some gray areas that need to be treated to get a clean mask.
I have intentionally not made the curve very steep to avoid oversharpening of the edge between white and black area.
Now Dodge and Burn tool becomes very handy. First we clean the white area of the mask along the edge. In the tool options we set under Type "Dodge“ and under Range „Highlights“:
Then we switch to Burn and Midtones to make the edge of the darker part of the mask black:
Note: Sometimes it is better not to darken the edge completely to avoid hard transitions!
After we have darkened the area along the edge of the mask we can paint the remaining part black. Now our mask is ready:
Now we need to turn our layer we just finished into a layer mask. To do this, we duplicate our original layer and add some layer mask from the menu. (Right click on layer and choose add layer mask). It doesn’t matter what kind of mask, we will replace it in the next step.
Activate the layer that we made above, copy it to clipboard (Ctrl-C). Click the layer mask of the duplicated layer and paste the copy from the clipboard (Ctrl-V – when Floating selection appears, click on Anchor layer).
Now the layer mask has been replaced by ours:
We don’t need the top layer any more and it can be deleted.
Now we darken the clouds and increase the contrast of the landscape and see how it looks like:
It looks good, but we have some fringing on the edge:
We can use Gaussian blur to soften the edge of the mask a little and then narrow the edge with levels.
Activate layer mask than Filters – Blur – Gaussian Blur. Size: 1-1,5 for x and y:
Than Colours – Levels. Move arrow for mid tones towards black until the white border disappears in the picture:
Now, the transition between clouds and landscape looks much better. However, there are still minor issues in some areas:
At this point the landscape shines through from the upper Layer that was darkened with the sky.
We could now extend or narrow the mask by one or two pixels, but at other places we would have changed the border as well.
Luckily we can do this locally by using the Warp Transform tool. We will now set the behavior of the Warp tool to Move pixels:
First we have to make sure that the layer mask is selected and then we just carefully pull the edge down a little bit with the brush. Now we have „pressed“ the edge of the mask down a bit.
As we can see, the edge is getting even darker now:
We have now pushed the edge of the layer mask underneath and narrowed it. That’s why we see even more edge of the dark landscape.
Now we also have to push that down, so to say to "hide it down“ behind the brighter landscape of the lower layer. So, we have to choose the layer itself (no longer the mask) and repeat the same step with Warp Transform brush – push edge down until we see no more fringing:
Now we have nice and smooth edge 
Ok, enough for now. More later