[header] How to address a large envelope [title] Put the envelope flap-side down. [step] Your envelope probably has a long flap that clasps or seals the contents inside. You don't want your address to be written on the same side as this flap. 
[title] Position the address in the center of the envelope. [step] While you don't need to take out a ruler and find the true center of the envelope, try to be as accurate as possible.