First, you need to fill in the recipient's mailing information in the center of the envelope. The first line is the recipient's name, the second line is the street address with a detailed house number, and the last line is the city, state abbr, and ZIP Code.
Second, write the sender's information on the left top corner of the envelope. If your mail cannot be delivered, it will return to the sender's address.
Finally, don't forget to stick the stamp on the right top corner. Otherwise, the letter will not go into the delivery process.
Here is the envelope example below.