I've looked in the wiki and forums and I have not seen this asked or answered directly. I understand the concept of a rare farm, and I have even used one, but I am not clear if I am doing it the right way. The way I did it was this:
I clicked on "Top 50", and when it randomly assigned me to an opponent deck, I just hoped that I would get one of the farm decks. But that means that a lot of the time, it is not a farm deck, and then I either quit that battle (and lose 15 electrum), or try out the battle anyway, just for practice. But it seems like some other people, when they make farm requests, are doing this some other way?