Mr Guthrie's Computing website
Add your lever and then put in redstone over the edges of the door as in the screenshot. I've put redstone laps at the bottom too, just in case you can't see that.
The torches aren't necessary, it was just getting dark.
You then need two redstone repeaters faced inwards. Set them on 3 and make sure you add four bits of redstone in the centre. I've also put a switch next to the redstone dust in the middle that will open and close the door. You can put this anywhere as long as there is redstone dust connecting it (though it will only work within about 7 or 8 blocks.
You need to set out your sticky pistons with 4 pistons each side facing inwards and then add a two high column of pistons facing to the entrance of the door on the rear 2 of the inward facing pistons
3. Redstone repeaters
4. Redstone and lever
2. Make the frame
All I have done is added a load of wood blocks around the pistons including ones in front of the forward facing pistons.
Build a hidden secret door with redstone
Here I will show you how to build a hidden door using redstone, redstone repeaters, sticky pistons and a lever.
Make sure that you have some sticky pistons (12), redstone dust (12), redstone repeaters (2), a lever and redstone lamps (4).