i) If you mean "fill out" as in writing the required details, then you write the address of the intended recipient clearly on the front. It may also be wise to write "From:" along with your name and address on the back at the top.
ii) If you mean "fill out" as in to stuff something full, then you simply put lots of things in the envelope.
Recipient Name
Recipient Address Line One
Recipient Address Line Two
Town / Region
City
County / State
Include Country if outside of that country
Postcode / Zipcode
The correct way to write the address on the front of an envelope is to start half way down and one third accross from the left (that's what I was told on a RSA course). Don't forget to write the post town in CAPITAL letters and use the postcode.