Things You'll Need
Instructions
Laying the Foundation
Dig a hole around the intended location of your door -- it will need to be ten blocks wide, five blocks long and two blocks deep as you face toward the future entrance.
Place a one block-tall line of dirt blocks down the center of the longest dimension of your hole. This will require ten blocks in total. Until you install the circuitry, every block you place will be on top of this line of dirt blocks.
Place another one block-tall line of dirt on top of the first, leaving out one block on either end. This will require eight blocks in total.
Assemble the Door's Structure
Place a one block-tall line of two dirt blocks on each side of the center foundation you have just created. This will leave you with a single empty space under each end if done correctly.
Place one sticky piston against each of those dirt lines, facing inward toward the center and each other. You can now attach whatever material you like to the face of these pistons to form the center of your door. This should leave you with a two block-wide gap down the middle at ground level.
Place a second sticky piston on top of each of the first two, also facing inward. Again, attach your material of choice to the face of the piston. Put a dirt block against the rear of each of these.
Place dirt blocks to form a bridge that leads you across the foundation pit between your pistons. It should be two blocks wide and two blocks long on each side. Make sure to leave the gap under this bridge rather than filling in your foundation.
Place the Circuit
Lay a line of redstone dust along the outer edge of the bottom of your foundation pit. Aside from the spaces inside your central wall, you are making a circle. Also lay redstone dust along the top of the dirt that makes up each side of your center wall. The wall should require only four dust in total if you followed the plan properly.
Place a redstone torch inside the one-block high gaps you left in the center wall. One torch will be installed on each side. At this point your pistons should shoot open and close the doorway.
Place two pressure plates on either side of your bridge directly above the redstone dust. Stepping on these should briefly open your new doorway and allow you to run through.