You can do this with your box blade. They won't be very deep but that may be ok. The idea is to tilt the box so that the edge on one side of the tractor is lower than the other.
Now if you drive down the road, go slowly, and lower the box it will cut the material on the outside of the tractor and pull it into the center of the road. Drive with the cutting edge where you want the ditch. Start with a small angle till you get the hange of it. You will find rocks, etc.. and this can be hard on the tractor unless you go slow.
A rear blade is a better tool IMHO, but you can get it done with the box blade.
Fred