The way I usually do it is draw a tiny version of the image with a stick figure first (i think this is called a thumbnail sketch????) and try to use two point perspective lines to plan where im going to place everything. When it comes to actually painting the piece, I usually use a really big messy brush to block in the general colors and shapes (so like the shapes of bushes, hills, trees) then I use more refined brushes to add details to that :D