It's fun and relaxing to do and it's can be as simple or as complex as you want it to be. There aren't many rules, just make sure the address to the person it is being sent to is easily read and that there is room for a stamp. Basically, that's pretty much it; your imagination can do the rest.
You can find the video I made creating my envelope art over HERE!
Again, working with an existing card from my stash to make the coordinating envelope.
Go ahead and make some mail art and don't forget the open call for the "All about me" mail art show.