This is definitely the week of mountain ranges. I had a question on reddit on how to draw mountain ranges that go east-to-west rather than north to south. Here’s a very quick tutorial on the difference. If you want to go deeper into the north-south version, check out this earlier tutorial on mountain ranges.
Note that I use a graphics tablet. You can do this with a mouse and low opacity brushes, but tablets are getting good and relatively cheap. I’d recommend picking up something like the Bamboo Splash if you’re going to be playing around with illustrating maps.