Step 1: Create the Door

Place the 9 blocks of your choice to make a 3x3 square.

Step 2: Place the On/Off Switch/Button

Place your switch/button two blocks away from the door to the left or right. I, for example, put mine on the left.

Step 3: Place the Command Blocks & Redstone

Look at the picture above. IT IS VERY IMPORTANT THAT YOU PLACE THE COMMAND BLOCKS, OR GATES, AND REDSTONE AS SHOWN IN THE PICTURE, however. If you are experienced with redstone then you might be able to edit the layout, but I do not recommend doing so.

Step 4: Code the Command Blocks

NOTICE: THIS IS THE HARDEST PART!

Start by going into every command block and typing "setblock".

Next, find the coordinates for every block of the wall and enter the coordinates into Section A & B.

Don't understand what I mean? Here's an example:If the bottom left block of the wall's coordinates were "X: 200 Y: 40 Z: 200" then I'd enter that into one command block in each section, so that the command in the two command blocks looks like this: "setblock 200 40 200".

Now, enter the id of the block of the wall into Section B.

For example, if my entire wall was going to be iron blocks then I'd put "42" or "minecraft:iron_block" into all of the command blocks in Section B. Then it should look something like this: "setblock 200 40 200 minecraft:iron_block"

Finally, in Section A do the same thing that I said in the example, but put "0" or "minecraft:air" instead of the block the wall is made out of.