It's been a long time since I had attempted to do something like this. I've done it in little ways, especially all of this past year, just not to remotely to this scale. The driving force is that this is a very common industry practice for getting ideas out to an art director very quickly, so It's something that I'm determined to be passable or even good at.
My first go at this sort of thing was a disaster, many times over. I didn't know particular functions of Photoshop well enough to get things to not be completely blown out. Fitting things in was very tough. My sense of scale was a lot worse (far from perfect still) and my perspective could get wonky very quickly. In short, my fundamentals were not up to par nearly enough to throw tricks at a board and expect it to look remotely good. Since those attempts, I've grown a ton, and with a job I'm determined to get looming off in the distance, I knew it was time to tackle this beast again. So I did, with my main focus simply being cohesion, not a polished super-correct piece. In that study, I do feel successful. Everything feels well-blended. There isn't a "this is a giant collage!" feeling when I look at it. As a final "piece" it needs a lot of work and time. This would be a good first pass to work off of with more reference for construction (I mainly just made parts work together, so it could be better served with reference and another go now that the main compositional elements are in there).
My next studies are those of existing landscapes and masses so that I can build a better visual library of how things are constructed. I've done this for people, animals, plants to a degree, a little bit for mechanical parts, but not a whole lot for environments. Clothing is also on the list. I was once very into costume design, but It's been over a decade now since then and that skill is both rusty and diminished.
Anyway, I'll get better at all of these things. I put the hours in for it, anyway, and try to stay as mindful as I can when I practice.