Stairs can be rafted from the crafting menu and takes 4x pile of planks and a foundation already on the ground. The stairs will snap into place on the foundation but cannot be rotated. Make sure not to fully upgrade the foundation the stairs are on, though, since you will not be able to use it if you do.
Stairs are great for reaching a second story or even further up. However, be careful, stairs are thin and steep, you can fall off of them if you're not cautious and you can take fall damage.