Start with a curved line to simulate the curve of the earth.
Draw a series of mountain peaks on the horizon.
Add a large river starting as a single line and getting larger as it gets closer.
Add more mountains in the mid ground.
Add some foreground ridges for depth and then add some trees. Don't draw every tree individually but hint at the vast forest. Less is more.
Download the printable guide to learn how to draw a Fantasy Kingdom to add to your landscape, and then finish things off with sunbeams and dramatic clouds.