Questions regarding how to build underground bunker

I want to create an underground bunker. My below method seems to not be the correct way. How would I go about doing this?

I have a simple 20x20x20 landscape and I added some stairs on the side of the landscape simulating going underground.

Next step was i was going to go to the Landscape > Add tool but I can’t add any landscape directly beside my stairs, it either covers the stairs or skips over one space and leaves a big gap between my stairs and the next landscape.