To make a door, open the crafting area made up of the 3x3 grid. Fill the entirety of the 1st and 2nd columns with 6 Wooden Planks. This recipe gives you 3 doors.
To make an oak door, place 6 oak planks in the 3x3 crafting grid. When making an oak door, it is important that the oak planks are placed in the exact pattern.
It's possible with only 18 redstone, 2 repeaters, 12 sticky pistons, and ONLY 20 blocks of any kind! My personal record for building this is 35 seconds!