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!