Instructions
Place blocks down using the right trigger to create your room. Leave an opening that is two blocks wide-by-two blocks deep-by-two blocks high.
Create three signs at your crafting table by combining six wooden blocks and one stick. Once you've created three signs, create three more.
Select a blank sign and use the left trigger to place it on the bottom left block on the inside of the entrance so it faces the bottom right block. Repeat the process for the top left block, the bottom right block and the top right block so each bears a sign facing the opposite sign. You can leave the signs blank.
Combine eight sticks and one wool block at your crafting table to build a painting. Aim the painting at the thin side of the bottom left sign you hung that faces the opening of the building. Press the left trigger to place the picture. It will cover the two-by-two-by-two opening.
Craft a door from six wooden blocks at your crafting table.
Walk through the painting and aim the cursor at the second set of blocks on the floor after the signs. Select your door in your inventory and press the left trigger to place the door on the block. Repeat with the second block in the entrance to create a set of double doors.