This is a picture of all the supplies you will need.
Arrange a sticky piston, and 2 stairs like this, so that the stair in contact with the piston is a corner stair.
Stack many of them on top of each other like so.
Fill the gaps in between so that the staircase is hidden in a wall.
Place down a lever in a reachable area.
Wire the lever to all of the pistons.
Build a platform for someone to stand there once they reach the top.
Enjoy your new hidden Minecraft staircase!