Make sure you use black permanent ink, whether you type or print addresses on envelopes. The black ink makes it easy for the postman to read the address, even if the envelope gets wet in any case. Moreover, the size of the letters should be big enough to be read if the envelope is held 3 feet away.
Put the envelope on a plain desk, with folding side facing down. Here, the longest side of the envelope should be at the top, in front of you. The delivery address should be mentioned near the centre of the envelope. You can start it around 1 inch to the left, and 1 inch to the top of the centre of the envelope.
First of all, you will write the name of the person to whom you plan to send the envelope. Always try to add Mr., Mrs., Ms., Dr. or any other honorary name, before mentioning the first and the last name.
In the second line, mention the street address. You can then use the third line to write down the name of city, followed by abbreviation of the state, followed by zip code of the area. If you do not know the zip code of the area, you can just Google it, as many websites offer zip codes for almost every part of the world. Cross check the zip code by checking on more than one website.
You can now enter your own address, the return address, at the top-left corner of the same side of the envelope. You can follow the same pattern, which you used for the addressee.