First to say that for almost everyone, display space is limited, so model farm fields/paddocks in model form are always rather overcrowded compared to 'real life'.
I should start off with some simple flat paddocks before trying hills. Cut some suitable size pieces of hardboard, and using wood glue, stick (on the rough side of the hardboard) some 'scatter', green for grass, sandy colour or whatever others of the available colours for paths, roads, etc. Look on eBay in the model train section for this. You should also find hedges and trees, although many will be too small for most farm toys. Look for scenery stuff intended for 'O guage' - the bigger train scale.
For hills, again start with hardboard as a base, and then stick suitably shaped expanded polystyrene pieces, perhaps with soggy paper ('papier mache') stuck over it. As this is white, first paint it matt green or brown, and then stick scatter on top.
I'll be back in a minute with a link to a useful forum.
Here it is: [You must be registered and logged in to see this link.]
As with most forums, you have to register to join to see most of it.
Be aware that most of the members there are mainly interested in model tractors and other agricultural machinery (& many members there are farmers in real life), and just like members here, many just display their models on shelves, but a lot do have model farm dioramas/layouts, and there are sections of the forum devoted to that. There is also a section for model/toy animals/people there, where you'll find most of my posts, but it is not a very active part of the farm toys forum - which is why I spend a lot more time here now.
So to be honest, you probably won't end up being one of their most active members, but I'm sure you'll find enough inspirations and friendly members willing to give helpful advice on how to make a model countryside.
There are probably also some model train layout forums worth searching for as well.