For the specifics of how to donate, you can contribute to the charity in one of three amounts - 25 Farm Cash, 55 Farm Cash or 240 Farm Cash. All donors, regardless of donation amount, will receive a one week permit to grow Daikons - you can plant as many as you'd like within that seven day period. Daikons cost 10 coins to play, reward you with 3 XP per square planted, and can be harvested in 6 hours. You'll receive 125 coins for each square harvested, making them a very profitable crop if you're looking to earn some coins fast, and you'll be helping a great cause in the process.
In addition, you'll receive a free collectible gift with your donation - a Japan Relief Flag which you can see at the top left of the image above.
Zynga and Facebook will donate 100% of all proceeds (meaning that Facebook is also donating their 30% cut of all Facebook Credit transactions) to support the Japan Earthquake Emergency Fund. There is no time limit as of this writing as to how long the Daikon will be available to purchase, so if you don't have the Farm Cash on hand right now, you'll likely have some time to purchase some before missing out on this opportunity to donate, and master a new crop all in one go.
Will you purchase a permit to plant Daikon in-game? Let us know in the comments.