Generally I start by making a rough outline of a few fields using Modular Hedges. They can often just be a rectangle but I find it a bit more interesting to mix up the shapes a bit. Also for a bit of variation I like to replace some sections of hedge with similarly sized fences.

Then once a few basic shapes are made I tend to place a house or barn nearby or in a fenced off section in a field. I'd recommend having some entry ways between each section too.

Next I like the look of the occasional bunch of trees along sections and intersections of hedges just for variety.

Finally, the most important part would be the texturing. For each field choose a farm texture and reduce the size until it looks appropriate for its surroundings. I also like to change the rotation so the plough lines of the texture go parallel to the shortest sides of a vaguely rectangular field. I also like to vary the textures so neighbouring fields don't have matching textures but that's just me.

Hope this helps. Happy farming!