Here is how to make a double redstone piston door
Dig a hole 6x4x2.
Put a block in each corner, and on top of that a redstone torch. Then connect the blocks the long way, and in the middle like the picture.
Fill in the floor.
Put a block on the redstone torches, and redstone dust on top of those blocks.
Put the sticky pistons next to each block with the redstone, and one more on top of each one. Then fill in the space between them with a block.
Completely hide the redstone, and you're done!