Choose a wall location for the secret door
Dig out a 2-block-high opening in the wall
Place sticky pistons around the opening
Connect the pistons to redstone dust
Add a hidden lever, button, pressure plate, or tripwire as the trigger
Test the mechanism to make sure the pistons extend and retract properly
Cover the pistons and redstone with matching blocks
Hide the trigger behind a painting, bookshelf, carpet, or under a block
Make sure the door blends in with the surrounding wall
Add a second layer of blocks if needed to conceal the mechanism