To properly address an envelope, the return address should be printed on the upper left side of the envelope. Start with senders name or Company name. On to the next line is the street address post office (P.O.) box number. Print the city name, then two space then, the two-letter capitalized state abbreviation and the ZIP code. The country is added if it's needed on the fourth line. For the Mailing address, simply follow the same step on the sender's information but this time in the center of the envelope.