First you want to put your Sticky Pistons four blocks away from each other and stack them with one on top of the other.
Next you want to put 4 Cobblestone in between the Sticky Pistons leaving a space in between.
Then put your next 2 Cobblestone on the outside of you Sticky Pistons.
Then on top of the 2 outside Cobblestone you want to put an Redstone repeater first then put one Redstone next to it.
After that you want to dig down 2 blocks make a hole under that second Cobblestone. In the hole under the second Cobblestone you put a Redstone torch in the center on both sides.
Then you want to put a Redstone repeater right in front of the hole with the Redstone torch. After that, take your ten Redstone and connect it right to the middle.
So now you want to put your Grass blocks right over your Redstone material to cover the hole.
After you cover your hole, you want to put two pressure plates exactly one block away from the 4 Cobblestone blocks.
Then when you step on the pressure plates the 4 Cobblestone blocks move away to create a space for you door!
And that's it! Very simple and easy to make! If you want to make the door open from both ways, just repeat the steps when you make the hole and add two more Pressure plates too! Hope you enjoy! :)