Choose a hidden location in a wall, floor, or mountain
Gather building blocks that match the surrounding area
Place sticky pistons in the shape of the door opening
Connect the pistons to a redstone circuit
Add a hidden trigger such as a lever, button, pressure plate, or tripwire
Cover the pistons and wiring with matching blocks
Test the mechanism to make sure the door opens and closes properly
Hide the trigger behind a painting, bookshelf, carpet, or other block
Add a second piston layer if you want a double-width secret door
Replace any exposed blocks so the entrance blends in with the environment