first off you need to dig a 7 block long hole in the ground ok
second you need to place 8 blocks down the left side of the hole
third place tnt in the hole except the very start and place 1 peice out of the hole at the end
then place some redstone going all the way down the right side of the hole and then redstone going ontop of the blocks on the left side but only 1 peice now put repeaters all the way down except for right before the last peice of tnt then place redstone make the dials on the repeaters at last tick and add water to the start of the hole put a lever and done.