When writing a letter envelope, it's important to follow some tried-and-true rules and etiquette guidelines. By doing so, you'll increase the likelihood that your letter makes it to its destination and that your recipient will be able to read it properly.
Whether you write your address on the front or back of an envelope, be sure to do so clearly and legibly. If you're using a pen, make sure it has dark ink and doesn't bleed. Ideally, you should also use a sans serif font, such as Helvetica, as this is easy for scanners and machines to read.
For the recipient's address, center it and include their name(s) on line one, their street address on line two, and their city, state and zip code on line three. If they live in an apartment complex, it's a good idea to add the apartment number on a separate line for clarity.
Finally, it's a good idea to write the recipient's telephone number in case of any issues with delivery. In addition, many people like to add a signature, either written with a pencil or even better, printed with a seal stamp, which is available at most stationery stores.