Choose a wall area for the secret door
Gather sticky pistons, redstone dust, redstone torches, levers or buttons, building blocks, and a door or blocks for the entrance
Dig out a 2-block-tall opening in the wall
Place sticky pistons on both sides of the opening facing inward
Attach building blocks to the front of the sticky pistons so they match the wall
Hide redstone wiring behind or beside the wall
Connect the pistons to a lever, button, pressure plate, or hidden switch
Test the mechanism to make sure the blocks move out of the way
Cover all visible redstone and piston parts with matching wall blocks
Add a hidden trigger such as a painting, bookshelf, floor plate, or lever behind a block
Adjust the timing if the door opens or closes incorrectly
Seal the surrounding area so the secret entrance blends into the wall