Placement
The address, return address and postage should all be on the same side of the package. (If it's improperly addressed, it might get returned to you).
Return Address
Print or type your address in the upper left corner of the label.
Special Handling
Clearly mark on the outside of the package if the item is fragile, liquid or perishable.
Postage
Use a stamp or printed postage in the correct amount.