Place two sticky pistons on each side of the doorway, facing inward
Place two more sticky pistons above or below them if making a 2×2 door
Attach blocks to the piston faces to match the wall
Dig a small space behind the wall for redstone wiring
Place redstone dust connecting all pistons
Add redstone repeaters if needed to extend the signal
Place a lever, button, pressure plate, or tripwire as the input
Connect the input to the redstone line
Test the door by activating the input
Adjust piston timing if the blocks do not move together
Hide the wiring behind walls or under the floor
Cover exposed redstone with blocks to finish the door