Step 1: Place and Activate the Pistons

 To begin, decide where you want your hidden door. The final apparatus will only take up about a 6 by 12 blocks. Place three sticky pistons spaced four blocks apart. 

Place your desired hidden door material in a two by four pillar between the pistons with a space between the hidden door and pistons.

To make the piston activators, place a redstone torch next to the bottom piston on either side. Then place a block above it, activating the second highest piston as seen above.

 To finish it up, place a piece of redstone dust on top the block you just placed, activating the final piston. Be sure to do this for both sides.