This is really simple with the backhoe, but you have to dig with the hoe facing straight back to get your side slopes even. First, dig you ditch to the depth and width that you want it. Then with the tractor is the same position, take off the edges with your bucket. With the tractor in the same spot, the hoe stick has to turn at an angle to get to the edges of the trench. The bucket needs to curl down, with the digging force starting at the width you want the slope to begin. The just dig down to the bottom of you ditch. In just a few attempts, it will make sense to you and you will be able to get that V or flat bottom V that you are looking for.
Eddie