Choose a wall location for the hidden door
Dig a 2-block-tall opening in the wall
Place sticky pistons around the opening
Put blocks on the piston faces to match the wall
Add redstone dust behind or beside the pistons
Connect the redstone to a lever, button, pressure plate, or hidden switch
Test the mechanism to make sure the pistons extend and retract properly
Cover the redstone and pistons with matching blocks
Hide the activation switch in a nearby spot
Walk through the opening when the pistons retract
Close the door by activating the switch again