You need to place four blocks of sand, then place two blocks of dirt between those four blocks of sand. This will result in a block of sponge. You may notice that the sand-dirt combination looks like a brick wall. This is because of the way Minecraft renders textures. If you want to see what your block looks like when placed correctly, simply type /render block @a ~ ~-1 ~-1 ~-3 ~-1 ~.