Oh I did explain it, but it’s a few posts below the one I linked. This should be the right one now, sorry.
I indeed did it without mods. It works just like MeowBeep says: You can draw roads to create pockets in the desired shape you wanna have your farms. Then, you simply fill these pockets with fields. Finally, you cut away the parts of the field that extends beyond the pocket by again drawing roads over those parts (then, these parts of the field get deleted) and delete these roads again. It’s really not difficult and it’s quite fun, actually. Try it yourself. 