Flipping the image is a very good thing because it's really gets you to watch every compositing detail.
The
law is simple here: the image should look in both ways perfect (if is
flipping horizontally). I flip the image all the time while I'm
working.

Oh
I found a detail here - and now I really know the story behind this
picture. It will something like lava mountain) I really refined the
image in the corner (this
rock basically) to define the
elements and the style of the picture. I think I'll consider that as a
something like standard element and style of my picture.

...Dropping some whites - really careful. It has to be a balance between lights and darks (mass and space) in this case.

It always good to drop a human figure for scale, when you are painting environments:
A
human with the dog: actually it's too big for my proportions. You can
just store this in mind and do it in several steps or so.

So
I really like how this part looks. Let's name it the 1st part. I will
be flipping the image - so this is for you to understand this faster.
So
for the 1st part - I really like it - the details and value, so the 2nd
part of the Image isn't really done - so let's do it.

Now it is very important do use lines (non masses) to define shape edges - some people sometimes forget to do this.

And finally some texture - I'll just go with simple dots.

So
this is for landscape black and white sketch. I'll definitely see you
in the next part of this tutorial where we will color this sketch and
bring it to final look.