Well since you said ‘write’ and not ‘type’, I’ll tell you the handwritten way to do this. Here’s a good example; (First Line, Name) Jackie Joe Doe, (Second Line, Address) 12523 Anywhere St. Apt 123, (Third Line, City State Zip) Austin, TX 73301. And that’s it! Make sure to include any apt or suite number, or it will never get there. If you want to use the zip+4 code you can look that up on USPS.com, but your mail will still get there if you don’t put that in.