How do I make stairs in Terraria? It's getting really annoying that NPC's can't go up platforms.
If you are looking for something that has the functionality like the platforms, where you can optionally go through them or up them, then that is not possible to make in a staircase.
With the changes in 1.2 though, everything should be able to walk up and down a 1 to 1 elevation change. So you can just make a tile of blocks that anyone can walk up and down. You can even use the hammer to make it look like a slope if you wanted, but this is optional and up to your taste in how you want it to appear.
Edit: interesting bit of new information from playing. You can place down Ingots of different metals and these act as a background as well as a foreground. How the AI will deal with them is a bit odd so far and I have not been able to rigorously test this, but these give you a set of stairs you can jump and land on or just simply walk past them. This may be a solution to your problem. I will update with more exact information as I find it, but it is a tentative third option.
As of version 1.2.3, if you place a Platform of any type, and hit it with a hammer (essentially making it a sloped platform), the platform will transform itself into a staircase. Here's the relevant line from the changelog for 1.2.3:
You can now turn platforms into stairs using the hammer.
Here's a video from YouTube demonstrating how this works.
There is now a faster way to build stairs without the hammer:
On the PC version, you can select a platform and hold down the "Ctrl" key. This will highlight a single block were the platform can be placed. If you place them in a diagonal instead of horizontal, it will automatically become stairs.
Not sure how it is done in xbox or other versions.