This double exposure tip is commonly used. It's very easy and the effects are always amazing. But I wish to include a small detail - allowing a few seconds to elapse before taking the second shot to allow it to be slightly different from the first shot.
Take your first shot. What are you looking for? The image should have high contrast, interesting patterns and shadows, preferably at the lower portion; which is where your second exposure will come out. Then flip your camera by making a 180º turn. Shoot your second shot. Clicking the shutter button may feel awkward but you’ll get used to it.
In the photos above, the second exposure was taken seconds after the first shot so the exposure below is not an exact duplicate of the first shot. If you are shooting a static landscape and are stationary, you can also get interesting effects like these photos.
In the end, it really depends how similar or different you want your doubles, just wait a bit before shooting the second exposure. It’s a question of style, preference and luck. Have fun and say “the world is flat” through your vertical flip doubles.