Step 1: Materials
2 Sticky Pistons
About any 2 stacks of any block
About 30 Redstone
Step 2: Make the Door
Make the door and walls this might take about 30 diamond blocks
Step 3: Redstone
Add some Redstone right from your lever (where the sticky pistons are) and make some blocks for the Redstone to touch the pistons
Step 4: Tryin It Out When It Is Done
Try it out when it looks like this.
Pull the lever and if the inside disappears and its replaced with diamond block it works.
Step 5: Now Make Your Room
CONGRATS! YOU HAVE FINISHED IT!