On the PC version, you can select a platform of any type, and hold down the Ctrl key, it will highlight a single block where the mouse is. You can then place the platform in a diagonal, and it will automatically become stairs. No need to switch to a hammer.

Not sure how it is done in xbox or other versions.

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.