Things You'll Need
Pen or pencil
Cardboard is a sturdy, lightweight and cheap craft material with many uses. Corrugated cardboard -- the type typically used in consumer boxes -- is made up of three layers of paper, involving a sheet of wavy "fluted" paper in soft vertical accordion pleats sandwiched between two smooth layers of paper. Whether you use cardboard to provide hidden strength to a project like a book cover, or it is the project's surface material, like a piece of cardboard origami furniture, the cardboard likely needs to be bent or folded. Because the material is designed to be stiff, a clean bend or fold can be difficult to achieve.
Bending Cardboard with the Grain
Lay your piece of cardboard on a hard work surface. Look at its edges. If you can see the wavy line of the fluted core of the cardboard you can cleanly bend or roll the cardboard in the same direction as the fluted material. You should be able to see little lines or ridges running along the surface of the cardboard. This is called its "grain."
Mark with a pencil or pen along the line where you want to bend the cardboard and fold along the line. The cardboard will easily fold along the fluted ridges in its core.
Line up your piece of cardboard so its grain is parallel to the edge of your work surface. Run the cardboard over the edge of a table, pushing down slightly along its length to crease the paper between each fluted ridge. You will now be able to roll the cardboard into a cylinder or bend it into an arch or "U" shape.
Bending Against the Grain
Lay your cardboard out on your work surface.
Draw your pattern onto the wrong side of the cardboard, marking any fold lines.
Line up your metal or metal-edged ruler along your fold line. Press the edge of the ruler firmly down into the cardboard. Lift up the ruler, move it down the line and press again if your fold line is longer than the ruler. The ruler will leave a groove in the cardboard without breaking its surface. The cardboard will now easily bend along the groove.