is there any special way of writing a letter to an inmate, like dos and donts?

The correspondence rules are similar for all detention centers (jails, prisons, penitentiaries), the letters may be and probably are read by the Correctional Officers. Unless this offender is on some mail restriction for things you should already know about (terrorist-type charges), mail is considered sacred and they don't censor much. You cannot speak of business issues like moving money around, buying/selling real estate, stocks or bonds. You cannot speak of escape or anything conveying surveillance of the institution. And you cannot speak of anything remotely criminal. You may speak of the inmate's criminal case, the inmate's defense or appeal. You may speak of current events or things of a relationship-nature. This shouldn't be too hard to follow.