...find the grid the terrain system is using...I usually start digging straight down until I hit the level, square 'bottom' of the hole, then I start horizontal digging from one of the sides for trenches. In the reverse, I then build UP the wall to the general height I need/want and then, I always do this, I 'flatten' the area before and behind where I'm currently standing.

Find the grid! Rounded corners get janky...perpendicular digging/building gets things 'straight'.