You have to send 'em (BCCI) a tweet along with your message for Sachin, and make sure you include a hash tag #ThankYouSachin in your tweet.
Your tweet should be like this:
@BCCI // Your Message // #ThankYouSachin
Once this done, you will receive a system generated reply sent from BCCI's official Twitter account, along with a tweet which includes Sachin's picture and autograph.
So, what are you waiting for? :)