Use sticky pistons to pull blocks away from an opening
Hide the piston mechanism behind walls, floors, or ceilings
Place a lever, button, pressure plate, or tripwire as the trigger
Use redstone dust to connect the trigger to the pistons
Add repeaters if the redstone signal needs to travel farther
Build a 2×2 piston door for a simple hidden entrance
Build a 1×2 piston door for a narrow secret passage
Use observers to detect block updates for automatic hidden doors
Use a bookshelf, painting, or wall decoration to conceal the entrance
Use trapdoors to make disguised floor or wall openings
Use slime blocks or honey blocks for moving hidden panels
Test the door repeatedly to make sure the blocks move correctly
Cover all visible redstone with blocks to keep the door secret