Measure and draw on your cardboard a rectangle of the desired length and width of your box, adding twice the height to each dimension. For example, if you want a box that is 10 inches long by 8 inches wide by 6 inches high, the rectangle should be 22 inches by 20 inches (10 + 6 + 6 and 8 + 6 + 6).
Measure and mark from each corner in both directions the height of the box. In this example, you would measure and mark 6 inches from each corner. You should have a total of eight marks.
Use the ruler to draw a straight line connecting the marks on the rectangle that lie directly across from each other. You should now have a rectangle the size of the length and width of your box centered inside the larger rectangle. If you extend the lines, you will have a square in each corner of the larger rectangle.
Cut out the large rectangle, which you had drawn first, with the utility knife.
Make a cut along one side of each of the corner squares with the utility knife, using the lines as a guide; don't cut into the inner rectangle. This will create tabs for the corner of the box. Score the other side of each square along the remaining line with the utility knife. You want to cut about a third of the way through the cardboard.
Score the line of each side of the inner rectangle in the same manner.
Fold each of the four sides up along the scores and fold the flaps around the sides of the box.
Attach the flaps with archival tape or PVA glue. If you use glue, clamp the tabs in place with binder clips while the glue sets. Allow to dry for about 30 minutes.
Repeat the above procedures to make the lid. Reduce the height in your measurements to 1 inch, or slightly more if you wish a larger overhang, and increase the length and width by 1/4 inch to accommodate the width of the cardboard.