To acquire the Secret Letter the player must have acquired the Unhappy Letter first. As the Unhappy Letter's description says the player must get someone to read it. To do this the player must mail it by putting in a postbox. After it is mailed, a letter will appear again in the mailbox at Home. There is only a small chance of of the Secret Letter being in the mailbox, otherwise another Unhappy Letter will be there.
The player must repeat the process until the Secret Letter shows up in the mailbox.