Find a viewpoint outdoors that gives you a reasonably interesting landscape in which there is an unbroken and clear horizon. View the scene through the camera and consider the different positions in which you could arrange the horizon line in the frame.
Now take a picture of each so that you end up with a short sequence in which the horizon is ranged from top to bottom.
I tend to position the horizon according to whether there is more foreground interest or a dramatic sky. In this example. I would emphasise the sky, so (reading from top to bottom), images 4 and 5 would be my preferred choices.