Step 1: Making Square Paper
Take one corner and fold it over to the other side. For neatness, you don't have to crease it. You're just looking for the opposite corner of your soon to be square sheet. Do this again for the other corner. You can use a pencil to mark where the fold went to.
Now line up the marks and fold along that line. Folding both a mountain and a valley fold along this line makes it easy to tear it cleanly. If you want to be more precise, you can use scissors and not fold anything at all yet.
Step 2: Making Guidelines
Step 3: Fold Corners In
Step 4: Making a Smaller Folded Square
Step 5: Unfolding
Unfold the right and left sidesout.
Now unfold the original top and bottom corners out. The paper should look like a tall hexagon.
Step 6: Prepping a Complex Fold
Step 7: A Complicated Fold
Press the top of the left and right sides to the top side.
The side should want to pull into a 3D shape now.
Step 8: Constructing the Sides
This should create a three sided box.
Step 9: Completing the Box
Your box should be finished, and hold itself together.
Step 10: Suggestions
To make a lid, just make another box, and fit one inside another. (It helps if you make the lid a little bigger or smaller than the original box. This can be done by starting with a smaller or larger square piece of paper at the beginning.)
If you want crisper edges, take time making sure creases are nice and tight.
To make a box more sturdy, try using different kinds of paper or glue to help hold the folds closed.
Decorative paper makes for decorative boxes!