Start with a standard 8.5"-11" paper.
Make a diagonal fold from one of the top two corners.
Cut or tear the excess that hangs out under the fold to get a perfect square.
Set the excess aside for now as we don't really need it.
Fold back across your original crease to make sure you got all the excess, as well as making sure your crease is good.
Make a fold across the other corner that you previously hadn't.
Open your page back up to see your nice big "X" across the center.
Now pinch the center of your pace to fold the sides of the X inward.
Make a crease across these folds.
Your box should look something like this.
Now, grab of of the loose flaps on the side.
pull it up to the top middle, line it up,
and make a crease that looks like this.
You're going to want to do this with all four sides
until you have something like this.
Now take the side center corners and fold them inward toward the center of the piece.
Again, you're going to want to do this with all four sides.
Until your box looks like this.
Now, take the loose flaps at the top
and push it into the envelope you've created
making a solid crease so that it will hold upon inflation.
Again do this for the rest of the three corners.
Now flip your creation over to see this small hole on the bottom.
If you gently blow air into this hole, your creation will inflate like a balloon.
And that's all there is to it.