It is fairly easy, provided you have a lot of luck. You must do the following:
1. Visit one of your neighbors farm in Farmville and help them ward off crows, moles, etc.
2. If you're lucky, you get randomly selected for Farmville and receive five bags of fertilizer. Consider the fact that everything is random and you can help 20 residents without the bags, as it may help one to find.
3. You know if you got five sacks of fertilizer Farmville if a small white bag and blue at the bottom left of the window of Farmville, who has the "x 5" sign near it.
4. If that appears, can now be used in the cultivation of their neighbors to fertilize. Just click on their crops and you fertilize (crops will shine once fertilized).
And that's all you have to do! Good luck and we all know when you manage to get the ribbon!