Hypothetically, if you are in prison and want to communicate with the outside world without being detected, what are some ways to hide a message within an innocuous letter?

Like, say, you are a revolutionary who's imprisoned, but you are allowed to write letters to your family. How would you covertly send a message to your fellow revolutionaries that only they can decode? (Your family are on your side will pass the letter along)

Obviously, it cannot appear to contain any obvious ciphertext, since the prison guards would just rip up your letter and not send it.