Although I haven't written a letter, there are many fans in official website, rf.com who wrote letters and signed them to share their experiences. Roger also published a mailing address in official website and official website, which goes something like this:
postal address
If you want to write me a letter, card or photo, please send it to the following address.
Please note that articles (balls, photos, etc.) received by mail for signature are not accepted and will not be returned.
Autographed card
If you want to order signature cards, we urge you to attach a self-addressed envelope (C6,162x114mm) and write the appropriate postage and your address on the envelope.
Please note that all letters without postage will not be returned.
You can include a special coupon for international mail or two dollars postage.
Please send envelopes, any cards or letters to the following address:
R.federerpostfach 4 103 Bottmingen
Switzerland
Thank you for your email!
When writing a letter asking for his signature, first write an envelope (size 162 x 1 14mm), write down your return address and put it in the letter sent to him. Just fold your reply envelope and put it in. Moreover, it is best that the return envelope should be at least as big as a postcard, and the return address can be written in Chinese, as long as the country name is written in English. Just write "People's Republic of China (PRC)" on the last line.
The letter you send can be simple and polite. You can put 2 dollars in it as postage. If you don't have $2, go to the post office and ask if there are any international postage stamps like this for sale. Federer official website also mentioned "international mail special coupons" in the above explanation, and this will do. ). It usually takes several weeks or 1-3 months after mailing, so wait patiently.
This is a sample of a letter I wrote to you. You can modify and personalize it before sending it:
Dear Roger: