What u need:
Two blocks
4 iron bars
1 flint and steel
2 netherrack
2 trapdoors

First U need to place two blocks (whatever u chose to be the sides) with two blocks inbetween, then place 2 netherrack inbetween (one lower)! It should be next to a wall or have something behind it. Because you need to place two trap doors above the middle blocks! You need to light the fire after that with flint and steel! Then place the four iron bars infront of it all!

Submitted by: EmmaBrine