first, dig down 2 x 2 and 1 block deep space
then make a 4 blocks tall wall with a block of your choice and 3 blocks wide like so
break the middle block of wall, and place dispenser facing out on both side
place a minecart, sign, and button on the wall that is NOT right next to space you dig down
break a block at the top of dispenser on both side and make a stair like structure on both side. the bottom block should go right behind the block where you placed button.
place a redstone dust on both sides like so
and then press a button to activate the door
from outside, right click through the lava and ride in minecart to get behind the lava
in order to go out, just press button and wait until lava goes down, and then go out
from outside, press the button again to put lava back and hide the door.
and that's it guys! hope you enjoyed tutorial and don't forget to leave a like above.