How do I form and pour long concrete steps for an exterior staircase?
You need to construct shuttering (usually timber) to hold the concrete in position while it cures. You'd usually have the shutter that forms the face of the stair "open" to allow concrete under to the lower step.

( This shows the principal )

If you use a stiff mix of concrete, it won't flow under the shutter forming the face of the stairs, but you will have to work harder to compact it to get a good finish. If you do find the occasional air void when you remove the shutter, you can fill in with a cement mix.