Start with the flat box on your table, with the wide part near you.
Fold the small tabs upward.
Then fold so the small tabs come together.
Fold one side up …
... and over.
Repeat on the other side ...
.… to complete the bottom.
Fold in the bottom flaps and tuck them inside the box.
Then fold the last two flaps and tuck them into the slots to close the box.
Now that you know how to assembly the box, learn how pros use the USPS envelope here.