up with 4 cacti on each sand line, per level.
go back to the front of your farm, and place your building material on top of the front cactus. place a second block directly behind that (going towards the back of the farm).
you will now need to break the block that is directly above the front cactus.
continue building a line across the top of the cacti. once you reach the last cactus in the row, place two blocks on top of it. next, place a block directly behind it (building towards the back of the farm).
you will now break every other block in the line you just created. use the cacti 