Sample letters to inmates exist online at sites such as spectator.org and clutchmagonline.com. Sample letters found on different websites serve different purposes, so it is up to you to get help from the one that suits your needs best.
The sample letter in spectator.org is a response to a letter from an inmate. It gives you a brief idea of some of the things you should say while responding to the mail. For example, it suggests that you should let the inmate know what is going on with the life he left.
On the other hand, clutchmagonline.com has a sample letter to a person who has been jailed for a second time. It shows some of the issues that you should highlight in such a mail, such as a message of hope.