Choose a wall location for the hidden door
Dig out a 2-block-tall opening in the wall
Place a sticky piston on each side of the opening, facing inward
Build the wall back around the pistons so they are hidden
Place the blocks that will act as the door in front of the pistons
Run redstone dust from both pistons to a hidden switch
Connect the redstone to a lever, button, pressure plate, or secret lever
Test the mechanism to make sure the pistons retract and extend properly
Cover all visible redstone with matching blocks
Add a hidden entrance trigger if needed, such as a painting, bookshelf, or floor switch